双真空主动型氢脉泽实验研究

介绍了一种为实现主动型氢脉泽小型化而设计的双真空结构氢脉泽的最新研究进展,并将其与上海天文台SOHM-4型主动型氢原子钟进行了比较。其物理尺寸和重量较原来有较大幅度减小,磁灵敏度和温度灵敏度指标均有所提高,长期稳定度和日漂移率优于预期。     

Environmental factors and hydrogen maser frequency stability

It is necessary to have a complete understanding of the environmental sensitivities of cavity-tuned hydrogen masers to obtain optimum frequency stability and to avoid common-mode frequency fluctuations. Measurements of environmental sensitivities (temperature, relative humidity, atmospheric pressure, line voltage and magnetic field) made at the National Institute of Standards and Technology have demonstrated that the frequency stability of a cavity-tuned, active hydrogen maser is not significantly degraded if the maser is operated in a moderately controlled environment. Under these conditions, common-mode frequency fluctuations caused by the observed environmental factors also are negligible.     

Spin-polarization effect on electron attachment to atomic hydrogen on liquid helium surface

No Heading Reaction rates of electron attachment to atomic hydrogen are measured as a function of magnetic field. The reaction takes place in a two-dimensional mixture of hydrogen atoms and electrons on liquid helium surface. Surface electron density, measured by using vibrating capacitor electrometer technique, decreases when H atoms are introduced. Applied high magnetic field suppresses electron attachment, H + e^– H^–, as well as hydrogen recombination, H + H H₂. Since the electronic state of negative hydrogen, H^–, is spin singlet, electron attachment is suppressed by spin-polarization. Possible microscopic mechanisms to explain the measured magnetic field dependence of the reaction kinetics are discussed.PACS numbers: 67.65.+z, 68.     

Frequency Stabilization of the He-Ne Maser

A bright and monochromatic radiation from an optical maser can be used as a stable standard of wavelength, when plane mirrors in the maser are automatically controlled so that the oscillation frequency can be kept very close to the center of the atomic line. The separation of mirrors is modulated at a low frequency with a small amplitude. The fundamental-, the second-harmonic, and the third-harmonic components of the modulation frequency in the light output give correction signals for the tilt of mirrors, the power level of excitation, and the separation of mirrors. The photobeat between two independently-stabilized masers of 1.15 microns filled with Ne20 and Ne22, respectively, has been observed. The observed fluctuation of beat frequencies shows a Gaussian distribution, and no systematic frequency drift has been found. It is found that the frequency of each maser stays within several parts in 1010 and the resettability is just as good. Some difficulties with stray magnetic field from magnetostriction coils and with earth magnetic field are found. Preliminary experiments on pressure shift and its effect on the stabilized maser are discussed. The theory has been developed, and the frequency deviations as functions of the gas pressure and of the amplitude of modulation have been calculated.     

A heterodyne laser system to study frequency stabilized Zeeman 633 nm He-Ne lasers deficient in temperature steadiness

A heterodyne laser system is constructed to study the effect of external transverse magnetic field on the dynamic instability of internal mirror TEM_00q He-Ne lasers having temperature instability. The system could provide precise detailed knowledge about the roles of both the temperature and the applied magnetic field separately. Two different internal mirror 633 nm TEM_00q He-Ne lasers with frequency stability of 10⁻⁶ are studied. The applied transverse magnetic field on He-Ne lasers increased the frequency stability to be in the order of ∼10⁻¹⁰, in spite of temperature instability. In the same time, a single-mode operation with an enhanced laser output power is obtained. The sensitivity of the method showed that the laser exhibits a short-term frequency stability of 4.7×10⁻¹⁰, which after one minute, when mode collapse starts, decreased to 2.6×10⁻⁸, over the next four minutes. This increase in the frequency difference of the inter-mode beat signal that affect the stability is attributed to the uncontrolled temperature of the laser tube. The results revealed that the magnetic field plays the dominant role to achieve the maximum frequency stability, while the unsteadiness of temperature of the laser tubes limits the long term single-mode laser operation.     

An investigation into the structure of aqueous NaCl electrolyte solutions under magnetic fields

Molecular dynamics simulations are performed to investigate the effects of magnetic fields with intensities of 1–10 T on aqueous NaCl electrolyte solutions at 298 K. The simulations employ the F3C (flexible three centered) water model and investigate electrolyte solutions with both low (1 M) and high (5 M) NaCl concentrations. The results show that the self-diffusion coefficient of the water molecules decreases in a low-concentration solution as the magnetic field intensity is increased, but increases in a high-concentration solution. The magnetic field enhances the mobility of the Na⁺ and Cl⁻ ions in both low- and high-concentration solutions. The average number of hydrogen bonds increases when the magnetic field is applied to pure water or to a solution with a low NaCl concentration, but decreases in a solution with a high-concentration. The results show that the enhanced mobility of the ions under a magnetic field causes serious damage to the hydrogen bond network in the high-concentration solution. Conversely, in the low-concentration solution, the structural behavior is dominated by the properties of the water molecules, and hence the hydrogen bonding ability is enhanced as the magnetic field is increased.     

Carrier-Density Dependence of Faraday Rotation and Spin Splitting in Cd1?xMnxTe

We employ spin-quantum-beat spectroscopy to investigate the carrier-density dependence of the spin-precession frequency and the magnitude of the Faraday rotation of Cd_1–x Mn _x Te samples at fixed magnetic field. We find an onset of saturation of the Faraday rotation at carrier densities as low as 4× 10¹⁶ cm^–3 and attribute it to electrons (not holes which dominate in other types of experiments). The spin splitting at fixed magnetic field remains density dependent down to 3 × 10¹⁵ cm^–3 (the lowest density accessible in our measurements) which suggests a direct influence of the carrier-density on the sp–d exchange not mediated by screening effects.     

一种基于星载氢钟误差信号提取的实现方法

针对中国新一代卫星导航系统的高精度定位要求,星载氢原子钟以极其优异的长期稳定性、漂移率以及温度特性,作为频率基准大规模应用于该系统.双频方案的被动型星载氢原子钟由物理部分及电路部分两大部分组成,微波激励信号对物理部分微波腔激励后产生了腔频误差信号及高稳晶振误差信号.对于如何提取并处理这两种误差信号,提出了一种对调幅信号...     

Magnetic Specific Heat of Heavily p-Type Doped (Zn,Mn)Te:P

The magnetic contribution to the specific heat of bulk crystals of Zn_1–x Mn _x Te ( x = 0.03) heavily (up to 10¹⁹ cm^–3) p-type doped with P is studied over the temperature range 0.5–15 K and magnetic field range 0–3 T. The magnetic specific heat observed at zero magnetic field indicates that a substantial part of the magnetic ions has the degeneracy of their magnetic ground state lifted by d–d and p–d exchange interactions. The effect increases for doped and annealed samples with higher concentration of conducting holes. We have also carried out a theoretical analysis that takes into account the contributions due to small magnetic clusters, single magnetic ions in crystal field of distorted crystal lattice, and low energy excitations of the p–d exchange-coupled system of local moments and carriers.     

Type ÉM2-15 vibration magnetometer for measuring the magnetic saturation moment of ferromagnetic specimens

Conclusion The type ÉM2-15 vibration magnetometer that has been developed makes it possible to measure the magnetic saturation moment of ferromagnetic specimens in a magnetic field having a strength of 480 kA/m over a temperature range from –150 to +300° C. The maximum measuring error of the magnetic moments in the ranges from 10·10^–6 to 2000·10^–6 A·m² is no more than 3%. The temperature measuring error is ±2° C and the temperature is maintained within ±1° C at any point in the range.Translated from Izmeritel'naya Tekhnika, No. 5, pp. 61–63, May, 1975.     

Stabilization of a laser radiation frequency

Conclusions The computations and experimental investigations thus carried out show that the use of an external resonator as a frequency discriminator of a laser provides it with a low-inertia automatic frequency control system. The design developed for a high-Q optical resonator provides it with a long-term frequency stability. The application of this model in a laser automatic frequency control system serves to obtain under complex conditions a short-term laser frequency stability better than 1·10^–8 and a long-term stability not worse than 4·10^–8 for 15 min.     

Thermal contact between cerium magnesium nitrate and liquid3He at very low temperatures

A new experimental arrangement has been used to measure the times for equilibrium between cerium magnesium nitrate (CMN) and liquid³He. Measurements have been made on a single crystal of CMN from temperatures of 5.5–15 mK and on a powdered specimen from temperatures of 2.5–35 mK. The thermal resistance deduced from the single-crystal data is proportional toT ^–2. The thermal resistance obtained from the powder data may be separated into three contributions: a resistance which is proportional toT ^–2in series with a parallel combination of a phonon resistance (with resistivity _P =3×10 ^–4 T ^–3sec·cm²·K ⁴ ·erg ^–1 ) and a magnetic resistance (with resistivity _M =9×10 ² T sec·cm ² ·erg ^–1 in zero applied magnetic field). A reduction in the magnetic resistance was observed when the powdered CMN was placed in a magnetic field, the magnitude of this reduction being comparable to that expected from a Redfield-like theory. TheT ^–2resistance is discussed in terms of two-dimensional energy transport within the CMN. The 17-Hz susceptibility of the powdered CMN was measured in magnetic fields of 0, 18.9, 30.2, and 37.8 G. The magnetic temperatureT* (H=37.8 G) was found to differ by +0.5 mK from the magnetic temperature in zero field whenT* (H=0)=3 mK.Work supported by the U.S. Atomic Energy Commission under Contract No. AT(04-3)-34, P.A.143.Supported by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under AFOSR Contract No. AFOSR/F-44620-72-C-0017.     

Collective modes and sound propagation in a magnetic field in superfluid3He-B

A high-resolution, ultrasonic (12–89 MHz) acoustic impedance technique has been used to investigate the order parameter collective modes in superfluid³He-B over a pressure range of 0–15 bar and in magnetic fields up to 180 mT. In agreement with earlier experiments, theJ=2 real squashing mode has been observed to split into five components in small magnetic fields. However, contrary to earlier theoretical estimates, the Zeeman shifts have been found to become extremely nonlinear as the magnetic field is increased. The extent of this nonlinearity is largest at low pressures and at temperatures close toT _c. In comparison with recent theoretical work, the nonlinear Zeeman shifts may be explained as a result of two effects. First, there is a significant distortion of the B-phase energy gap in large magnetic fields. Second, there is an important coupling between the sameJ _zsubstates of the differentJ modes. In this sense the nonlinear evolution of the real squashing mode constitutes the observation of the Paschen-Back effect in³He-B. A comparison of the observed Zeeman shifts with theoretical expressions has yielded information about particle-particle and particle-hole interaction effects in the superfluid. In the limitT 0 and above a threshold field, the real squashing mode has been found to possess additional structure. TheJ _z=0 substate has been observed to split into a doublet. The separation between the two components of the doublet is of the order of 100–200 kHz and remains independent of the magnetic field. The origin of the doublet may be understood in terms of a recent theory which postulates a texture-dependent collective mode frequency. Further, at extremely small fields the effects due to dispersion of the real squashing modes have been found to be important. The magnitude of the dispersion-induced mode splitting in zero field is found to be consistent with theoretical predictions. TheJ=2 squashing mode has also been studied in the presence of a magnetic field. TheJ _z=0 state of the squashing mode is observed to shift to lower temperatures in a magnetic field. An additional field dependence of the observed acoustic impedance is interpreted as the evolution of theJ _z=–1, –2 states, but appears to be inconsistent with theoretical predictions.     

Pulsed Fourier-Transform Ultrasonic Spectroscopy for Ultralow Temperature Applications

Pulsed Fourier-Transform (FT) ultrasonic spectroscopy has been developed for the purpose of studying ³ He at ultralow temperatures. Our method is analogous to pulsed FT NMR and possesses several advantages over conventional time-of-flight acoustic techniques. For use at low temperatures, the most significant advantage is the ability to observe the frequency spectrum while the temperature, pressure, and magnetic field may be independently tuned. More specifically, using longitudinal LiNbO₃ transducers operating both on- and off-resonance, we are able to study several broadband windows of frequency, namely 16–25 MHz, 60–70 MHz, and 105–111 MHz. The determination of the energy gap of ³ He-B is presented as an effective application, and other novel uses are discussed.     

Physics of systematic frequency variations in hydrogen masers

The frequency stability of hydrogen masers for intervals longer than 10(4) s is currently limited by systematic processes. The physics of frequency-determining mechanisms internal to the maser that are susceptible to systematic variations, and the connections between these internal mechanisms and external environmental factors are discussed. From estimates of the magnitudes of systematic effects, it is found that the primary internal mechanisms limiting long-term maser frequency stability are cavity pulling, at the level of parts in 10(15) per day, and wall shift variations, at the level of parts in 10(16) to parts in 10(15) per day. Strategies for reducing systematic frequency variations are discussed.     

Experimental Study of the Frequency Stability of a Maser Oscillator Operated with an External Feedback Loop

In this paper, we report the results of an experimental study of the short-and mid-term frequency stability for both a hydrogen maser and a rubidium maser operated with an external feedback loop to modify the cavity quality factor. A revised version of the theoretical expression of the frequency stability for this type of maser is given and a numerical solution for various maser parameters is calculated. The predicted frequency stability exhibits an optimum when the cavity Q is varied. The experimental results presented in this study agree with this conclusion.     

Cyclotron Resonance of Composite Fermions

The introduction of suitable fictitious entities occasionally permits to cast otherwise difficult strongly interacting many-body systems in a single particle form. We can then take the customary physical approach, using concepts and representations which formerly could only be applied to systems with weak interactions, and yet still capture the essential physics. A most notable recent example occurs in the conduction properties of a two-dimensional electron system (2DES), when exposed to a strong perpendicular magnetic field B. They are governed by electron–electron interactions, that bring about the fractional quantum hall effect (FQHE). S. Das Sarma and A. Pinczuk (eds.), Perspectives on Quantum Hall Effects (Wiley, New York, 1996). Composite fermions, that do not experience the external magnetic field but a drastically reduced effective magnetic field B*, were identified as apposite quasi-particles that simplify our understanding of the FQHE. J. K. Jain, Phys. Today, 39–45 (2000). J. K. Jain, Phys. Rev. Lett. 63, 199–202 (1989). They precess, like electrons, along circular cyclotron orbits, with a diameter determined by B* rather than B. B. I. Halperin, P. A. Lee, and N. Read, Phys. Rev. B 47, 7312–7343 (1993). O. Heinonen, (ed.), Composite Fermions (World Scientific, Singapore, 1998). R. R. Du, H. L. Stormer, D. C. Tsui, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 70, 2944–2947 (1993). R. R. Du et al., Phys. Rev. Lett. 75, 3926–3929 (1995). R. L. Willett, R. R. Ruel, K. W. West, and L. N. Pfeiffer, Phys. Rev. Lett. 71, 3846–3849 (1993). V. J. Goldman, B. Su, and J. K. Jain, Phys. Rev. Lett. 72, 2065–2068 (1994). J. H. Smet, D. Weiss, R. H. Blick, G. Lütjering, and K. von Klitzing, Phys. Rev. Lett. 77, 2272–2275 (1996). The frequency of their cyclotron motion remained hitherto enigmatic, since the effective mass is no longer related to the band mass of the original electrons and is entirely generated from electron–electron interactions. Here, we demonstrate the enhanced absorption of a microwave field that resonates with the frequency of their circular motion. From this cyclotron resonance, we derive a composite fermion effective mass that varies from 0.7 to 1.2 times the electron mass in vacuum as their density is tuned from 0.6× 10¹¹/cm² to 1.2× 10¹¹/cm².     

蓝宝石主动型氢原子频标

研制了采用蓝宝石加载微波腔的蓝宝石主动型氢原子频标,腔体外径18 cm,高度20 cm,重量约2.5 kg。由于微波腔体积的减小,整钟体积较传统大氢钟明显减小。经中国计量科学研究院测试,该蓝宝石主动型氢原子频标的技术指标为3.0×10-13@1 s、3.8×10-15@1 d,是目前国内体积最小的高指标、高可靠主动型氢原子频标,可在实验室、车载等环境下使用。     

脉冲微波式氢原子钟的研究进展

在传统被动型氢原子钟基础上,利用复杂可编程逻辑器件(CPLD)控制数字衰减器产生两个相干脉冲微波,激励氢原子发生Ramsey干涉,从而压缩原子跃迁谱线宽度,提高被动型氢原子钟稳定度性能。利用单片机控制直接数字频率合成器(DDS)产生扫频信号,可以得到完整的Ramsey条纹。目前已观测到Ramsey条纹并成功将原子跃迁谱线宽度压缩至1.2 Hz,仅为传统被动型氢原子钟线宽的40%。     

Vibrons and Phonons in Solid Hydrogen at Megabar Pressures

Raman and infrared active vibrons and phonons in the solid molecular hydrogen at megabar pressures have been studied by the first–principles band theoretical calculations assuming the hcp lattice of the centers of the molecules. The frequencies of the out–of–phase stretching motion of the two molecules in the unit cell are higher than those of the in–phase stretching motion. Pressure dependence of the frequency of a phonon–like mode, which lies in 1300 cm ^–1 ~ 1900 cm ^–1 at pressures 70 GPa ~ 200 GPa, is in good agreement with the recent experiments. Our results suggest that the lattice of the solid hydrogen remains hcp beyond the phase transition at 150 GPa.