Acceleration of cold Rb atoms by frequency modulated light pulses

The displacement of Rb atoms in a magneto-optical trap (MOT) caused by the force of a finite time series of counter-propagating frequency modulated light pulse pairs is measured as a function of the chirp of the pulses. The frequency modulated light pulses induced ⁸⁵Rb 5²S_1/2 F=3 ↔ ⁸⁵Rb 5²P_3/2 F'=2, 3, 4 excitation and de-excitation of the atoms. The result of this excitation de-excitation process is a force causing the acceleration and, consequently, the displacement of the maximum of the spatial distribution of the trap atoms. The time dependence of the populations of the levels of the atom are calculated — including also the ⁸⁵Rb 5²S_1/2 F=2 and F'=1 states — as the result of the interaction with the finite train of counter propagating frequency modulated light pulses by the solution of the Bloch equations. As the result of the measurement the interval of the chirp of the frequency modulated light of given intensity where the transitions take place, are determined. The results of the experiment and the expectation on the basis of model calculations are in qualitative agreement.     

Crystal structures of Ni2MnGa from density functional calculations

The different crystal structures of ferromagnetic Ni₂MnGa have been calculated using density functional theory (DFT) with special emphasis on the modulated structures 10M and 14M. These are important for understanding the stability of Ni₂MnGa martensites and their functionality as shape-memory materials. The modulated structures have been optimized in the calculations and their properties are discussed in relation to the structures without modulation. The occurrence of the modulated structures is related to the soft TA₂ phonon mode observed in Ni₂MnGa. The latter is related to the specific nesting behaviour of the Fermi surface in Ni₂MnGa. Particular shapes of the modulated structures are stabilized by the covalent interaction mediated by the p-orbitals of Ga and d-orbitals of Ni. The role of this interaction becomes clear when considering the phonon dispersion spectrum of Ni₂MnGa, where characteristic anomalies occur in the coupling of acoustical vibrational modes and the optical modes of Ni.     

Unusual sequence of commensurate-incommensurate phase transitions observed in KLiSO4

An unusual structural phase transition sequence is observed in KLiSO₄ by EPR: high temperature prototype phase -incommensurately modulated phase - commensurately modulated phase - incommensurately modulated phase - low temperature prototype phase (reentrant phase). The low temperature phase has the same symmetry C⁶₆ of the high temperature prototype phase. The main feature of the modulation are rotations of the sulphate tetrahedra, which were precisely determined in the commensurately modulated phase.     

Commensurate and incommensurate modulated structures of Rb2ZnCl4

The commensurate and incommensurate structures of Rb₂ZnCl₄ have been refined using the four-dimensional formalism for modulated structures. They are characterized by a rigid body modulated rotation of the ZnCl₄ tetrahedra and by a translation motion of the Rb atoms.     

Ultrafast Time-to-Two-Dimensional-Space Conversion System Using SHG Crystal

We propose an ultrafast optical time-to-two-dimensional (2-D) space conversion system using a crystal of β-BaB₂O₄. The proposed system can optically retrieve a 2-D spatial distribution (an image signal) from an ultra-short laser pulse modulated by an image. The fundamentals are the time-to-space transform and the spatial time-frequency transform. Optical time gating using a nonlinear optical crystal is applied for the time-to-space transform. The experimental system can convert modulated temporal signals into 2-D spatial distribution at the bit rate of 8 Tbit/s.     

Amplitude modulation of the cw laser light in the TEM mn mode by means of a mechanical chopper

Amplitude modulation of the laser beam in a TEM _mn mode by means of a mechanical chopper is investigated on the basis of the chopping model represented by a moving system of infinitely long, parallel slots and mark spaces. The cases of rectangular and axial symmetry of the laser beam are both treated. The explicit expressions for the waveform of the modulated normalized transmitted laser power are deduced and their consequences investigated. It is found that in the case of rectangular symmetry, unlike the case of axial symmetry, the TEM _mn modes give, for a constant value of the mode numbern and for any value ofm, the same time dependence of the amplitude modulated laser power. The notion of the equivalent modulation widths is introduced and conditions for the efficient amplitude modulation are found.     

Effects of signal modulation and coloured cross-correlation of coloured noises on the diffusion of a harmonic oscillator

The diffusion in a harmonic oscillator driven by coloured noises $\xi(t)$ and $\eta(t)$ with coloured cross-correlation in which one of the noises is modulated by a biased periodic signal is investigated. The exact expression of diffusion coefficient d as a function of noise parameter, signal parameter, and oscillator frequency is derived. The findings in this paper are as follows. 1) The curves of d versus noise intensity D and d versus noises cross-correlation time $\tau_3$ exist as two different phases. The transition between the two phases arises from the change of the cross-correlation coefficient $\lambda$ of the two Ornstein--Uhlenbeck (O-U) noises. 2) Changing the value of $\tau_3$, the curves of d versus Q, the intensity of colored noise that is modulated by the signal, can transform from a phase having a minimum to a monotonic phase. 3) Changing the value of signal amplitude A, d versus Q curves can transform from a phase having a minimum to a monotonic phase. The above-mentioned results demonstrate that a like noise-induced transition appears in the model.     

Studying acoustic-wave fields in langasite-family crystals using the BESSY II synchrotron radiation source

The process of the excitation and propagation of pseudosurface acoustic waves in crystals of the langasite family is studied via X-ray diffractometry for the first time. The investigations are carried out using the BESSY II synchrotron radiation source in the double-crystal X-ray diffractometer scheme. The process of pseudosurface acoustic wave propagation is studied based on an analysis of the diffraction spectra of acoustically modulated crystals. Both the velocities of the pseudosurface acoustic waves and the power flow angles of the acoustic energy are measured for the first time. The pseudosurface acoustic wave is shown to be flowing. Surface and pseudosurface acoustic waves generated by an interdigital transducer in the Z cut of a La₃Ga_5.5Ta_0.5O₁₄ crystal are compared.     

Effects of a magnetic field on a modulated phase

The effects of a magnetic field on a modulated phase are studied. A modulated phase is found to have two critical fields H₁ and H₂. For a large enough magnetic field, H₁ and H₂ can be approximated by a linear law. As a result, the minimum magnetic field needed to destroy a modulated phase is a constant. The minimum magnetic field also greatly depends on the order of a commensurate phase. A very high order commensurate phase and an incommensurate phase cannot survive a magnetic field. The behaviour of a magnetoelastic chain in a magnetic field can be described by a harmless devil's staircase. The inverse temperature is found to play a role similar to that of a special magnetic field. The deeper physics underlying these new phenomena is the breaking of the left-right symmetry of a phase diagram. As a result a controllable path to a modulated phase is found.     

KCl:OH-中O2-色心实验研究

通过调制吸收光谱和电子自旋共振(ESR)谱的测量得知,在被高能射线辐照前后,KCl:OH^-中氧的成分主要以O₂^-的形式存在。O₂^-在样品中的浓度随辐照时间增加而增加,直至(OH)^-完全被辐照分解。O₂^-有相当强的稳定性。其存在与产生过程对增加F₂⁺激光色心的稳定性无直接作用。本文还根据ES 关键词：     

Interaction of frequency modulated light pulses with rubidium atoms in a magneto-optical trap

The spatial displacement of the ⁸⁵Rb atoms in a Magneto-Optical Trap (MOT) under the influence of series of frequency modulated light pulse pairs propagating opposite to each other is measured as a function of the time elapsed after the start of the pulse train, and compared with the results of simulations. Adiabatic excitation and consecutive de-excitation take place between the ground 5²S_1/2 (F=3) and the 5²P_3/2 (F'=2, 3, 4) excited levels as the result of the interaction. The displacement of the ⁸⁵Rb atoms is calculated as the solution of simple equation of motion where the expelling force is that arising from the action of the frequency modulated light pulses. The restoring and friction forces of the MOT are taken into account also. The system of Bloch equations for the density matrix elements is solved numerically for transitions between six working hyperfine levels of the atom interacting with the sequence of the frequency modulated laser pulses. According to these simulations, the momentum transferred by one pulse pair is always smaller than the expected 2ħk, (1) where ħ is the Plank constant and k=2π/λ where λ is the wavelength, (2) having a maximum value in a restricted region of variation of the laser pulse peak intensity and the chirp.     

Phase diagram of lithium-doped copper oxide, Cu1−xLixO

Heat capacities of lithium-doped samples of CuO have been measured below room temperature by adiabatic calorimetry. The antiferromagnetic ordering transition to incommensurately modulated state was detected as a step in the heat capacity. Its concentration dependence was compatible with existing reports based on Li-NMR. The incommensurate-commensurate transition of lithium-doped copper oxide was clearly detected for the first time. The magnetic phase diagram of Cu_1−xLi_xO was thus constructed. The suppression of both transition temperatures by the Li doping is nearly twice as strong as that expected from mean-field and percolation theories.     

聚苯胺基固态染料敏化太阳电池中电子输运性能的研究

以导电聚苯胺为空穴传输材料，制备了固态染料敏化太阳电池(DSC).利用强度调制光电流谱(IMPS)和强度调制光电压谱(IMVS)研究了TiO₂多孔膜内的电子输运及复合过程.通过TiO₂多孔膜内电子的平均传输时间(τ_d)和电子寿命(τ_n)及对IMPS实验数据的拟合，获得电子在TiO₂膜内的有效扩散系数(D_n)和扩散长度(L_n).这些聚苯胺基电池中的τ_n值为相应的液体型电池的1/10倍左右，表明在该固体电池中存在严重的光生电子的复合过程，这很可能主要是与氧化态染料分子和导电电子间的复合有关.随着TiO₂膜厚的增加，τ_n和τ_d均变小，但D_n和L_n随之增加，只有在合适的膜厚范围内才能获得较高的光伏性能. 关键词： 聚苯胺 染料敏化太阳电池 IMPS IMVS     

Magnetic-field-induced orientational phase transition in a nonuniformly stressed iron borate single crystal

Spatially nonuniform magnetic anisotropy was induced in the basal plane of an iron borate (FeBO₃) single crystal by applying low-symmetry stresses. The effect of nonuniform magnetic anisotropy on the magnetic state of this weak ferromagnet was studied by magnetooptic methods. It is revealed that, when a nonuniformly stressed FeBO₃ crystal is magnetized in the basal plane along a certain direction (depending on the symmetry of the applied stress), a transition from a uniform to a spatially modulated magnetic state occurs, which is not observed in the crystal in the absence of stresses. The modulated magnetic phase of the crystal can be represented as a static spin wave linearly polarized in the basal plane, with the azimuth of the weak-ferromagnetism vector oscillating about the direction of the mean magnetization. The temperature and field dependences of the spatial period of the modulated magnetic structure and the amplitude of oscillations of the ferromagnetism vector are studied, and the temperature dependence of the field range over which the modulated phase exists in the nonuniformly stressed FeBO₃ crystal is found. The results are discussed in terms of the phenomenological theory of magnetic phase transformations. It is shown that the theoretical model used consistently describes all the experimental results of the study of the noncollinear magnetic phase of the nonuniformly stressed FeBO₃ crystal.     

A new method to excite magnetoacoustic waves and resonances

Magnetoacoustic waves and resonance behaviour, excited by an amplitude modulated rf field (ω_rf ? ω_AM) in a plasma cylinder, were observed. The modulated microwave power was coupled to the plasma by means of a simple slow-wave structure.     

Detection of glutamate and glutamine (Glx) by turbo spectroscopic imaging

Turbo spectroscopic imaging (TSI) is a spin echo spectroscopic imaging technique in which two or more echoes are acquired per excitation to reduce the acquisition time. The application of TSI has primarily been limited to the detection of uncoupled spins because the signal from coupled spins is modulated as a function of echo time. In this work we demonstrate how the TSI sequence can be modified to observe spins like the C₂ protons of Glx (≈3.75 ppm) which are involved solely in weak-coupling interactions. The technique exploits the chemical shift displacement effect by employing TSI refocusing pulses that have bandwidths which are less than the chemical shift difference between the target spins and the spins to which they are weakly coupled. The modified TSI sequence rewinds the J-evolution of the target protons in the slice of interest independently of the echo time or echo spacing, thereby removing any signal variation between successive echoes (apart from T₂ relaxation effects). In this study we tailored the narrow-bandwidth TSI sequence for observation of the C₂ Glx protons. The echo time was experimentally optimized to minimize signal contamination from myo-inositol, and the efficacy of the method was verified on phantom solutions of Glx and on brain in vivo.     

Nuclear quadrupole resonance exchange spectroscopy with shaped radiofrequency pulses

Recent work on the nuclear quadrupole resonance (NQR) investigation of molecular dynamics in the solid state has relied on 2D methods. We report our studies of dynamic processes by 1D shaped pulse NQR spectroscopy. Significant advantages include considerably shorter experimental duration, clear definition of the exchange time window, and avoidance of off-resonance effects. The reorientation of the Cl₃C? group in polycrystalline chloral hydrate [Cl₃C–CH(OH)₂] is considered as a test case. This may be modelled as a three-site exchange process. An analysis of the generalised Bloch–McConnell equation is performed to formulate the kinetic matrix. The present approach involves simultaneous excitation of the sites that undergo chemical exchange by employing a suitably modulated shaped RF pulse, followed by a mixing time, and finally a suitable read pulse for signal detection. The experimental signal intensities are plotted against the mixing time to extract the kinetic parameters, i.e. the exchange rate and the spin-lattice relaxation rate. Variable temperature measurements are carried out to determine the activation parameters. Short experiment times are possible in our 1D mode, enabling a large number of runs to be readily performed as a function of mixing time and temperature. The kinetic and activation parameters obtained in the case of chloral hydrate are in good agreement with recent literature values.     

电极表面改性对染料敏化太阳电池性能影响的机理研究

采用强度调制光电流谱(IMPS)和强度调制光电压谱(IMVS)技术,从染料敏化太阳电池(DSC)电子传输和复合角度对比了不同光强下导电玻璃表面阻挡层及TiO₂薄膜优化使电池性能改善的内在原因.阻挡层的引入和TiO₂薄膜的优化均通过电沉积法实现.结果表明,对多孔薄膜电极的不同改性均提高了电池的短路电流J_sc和效率η,但对电子传输和复合过程的作用机理有所不同:前者延长了电子寿命τ _n,但电子传输时间τ _d变化不明显;而后者则主要是延长τ _n的同时也缩短了τ _d. 关键词： 染料敏化 太阳电池 调制光电流谱/调制光电压谱 电子输运     

Low-temperature phase transition in Ba2TiGe2O8

Previous work on Ba₂TiGe₂O₈ crystals has shown an unusual low-temperature (～ 223 K on cooling, ～ 273 K on heating) phase transition. Precession x-ray photographs on Ba₂TiGe₂O₈ single crystals show an incommensurate modulation along b*, and, for the first time, also along a*. Single crystal intensity data confirm the average structure in space group Cmm2. There is positional disorder in the pyrogermanate groups, and this is the probable cause for the modulated structure. The low-temperature phase transition is proposed to be a lock-in transition, with the modulation along a* locking in at a value of 1/3. Several properties, as well as other unusual features of the low-temperature phase transition, are discussed in light of the proposed lock-in transition. Domain studies show that the ferroelastic domains are unstable in the low-temperature phase.     

The optical properties of two-dimensional Scarff parity–time symmetric potentials

We investigate the optical properties with two-dimensional (2D) Scarff parity–time (PT) symmetric potentials, including linear case, and self-focusing and self-defocussing nonlinear cases. For linear case, the PT-breaking points, the eigenvalues and eigenfunction for different modulated depths of 2D Scarff PT symmetry complex potential are obtained numerically. The PT-breaking points increase linearly with increasing the real part of the modulated depths of PT potential. Below the PT-breaking points, the eigenvalues of linear modes are real, however, eigenvalues of linear modes are complex above the PT-breaking points. For nonlinear cases, the existence of fundamental and multipole solitons is studied in self-focusing and self-defocussing media. The eigenvalue for linear case is equal to the critical propagation constant _bc$b_c$ of soliton existing.