Two-color coherent population trapping in a single Cs hyperfine transition,with application in magnetometry

We investigate the coherent population trapping effect occurring between the Zeeman sublevels of a given hyperfine state of Cs when excited by a single-mode diode laser, which is frequency modulated in the kHz–MHz range. In the presence of a dc magnetic field, simultaneous excitation of pairs of ground-state magnetic sublevels to common excited magnetic sublevels is performed. As a result, coherent population trapping resonance is detected at only a given modulation frequency, whose value gives a measure of the magnetic field. The parameters of the resonances are measured in order to determine the ultimate accuracy of the magnetic field measurement. Received: 6 December 2002 / Revised version: 24 February 2003 / Published online: 2 June 2003 RID="*" ID="*"Corresponding author. Fax: +39-0577/23-4689, E-mail: biancalana@unisi.it     

Time-resolved photoemission electron microscopy of magnetic field and magnetisation changes

Owing to its parallel image acquisition, photoemission electron microscopy is well suited for real-time observation of fast processes on surfaces. Pulsed excitation sources like synchrotron radiation or lasers, fast electric pulsers for the study of magnetic switching, and/or time-resolved detection can be utilised. A standard approach also being used in light optical imaging is stroboscopic illumination of a periodic (or quasi-periodic) process. Using this technique, the time dependence of the magnetic field in a pulsed microstrip line has been imaged in real time exploiting Lorentz-type contrast. Similarly, the corresponding field-induced changes in the magnetisation of cobalt microstructures deposited on the microstrip line have been observed exploiting magnetic X-ray circular dichroism as a contrast mechanism. The experiment has been performed at the UE 56/1-PGM at BESSY II (Berlin) in the single-bunch mode. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-6131/392-3807, E-mail: krasyuk@mail.uni-mainz.de     

An all-optical, high-sensitivity magnetic gradiometer

Optical magnetometers have reached sensitivities that make them interesting candidates for the measurement of weak magnetic fields also outside physics laboratories. In order to overcome problems with stray magnetic fields a common solution with existing magnetometers is to operate a pair of them in a gradiometer configuration: one sensor measures the signal plus the stray fields, while the other one is mounted such that it is only influenced by the stray fields. In the difference signal the stray fields cancel. We have constructed such a gradiometer consisting of two sensors based on coherent population trapping (CPT) resonances in a thermal cesium vapor. Using a magnetic bias field the intrinsically scalar CPT magnetometer can be turned into a true vector magnetometer that is insensitive to magnetic fields perpendicular to a chosen measurement direction. We describe how to align and calibrate the gradiometer. Stray field suppression by more than two orders of magnitude has been achieved, limited by the sensitivity of the magnetometer. This makes possible the detection of picotesla flux density changes in a weakly shielded or even unshielded environment. Received: 3 April 2002 / Published online: 15 November 2002 RID="*" ID="*"New address: Département de Physique, Université de Fribourg, Chemin du Musée 3, 1700 Fribourg, Switzerland RID="**" ID="**"Corresponding author. Fax: +41-26/300-9631, E-mail: robert.wynands@unifr.ch     

Grooving an alumina surface as a means to inhibit secondary electron emission under grazing incidence

We observe charge multiplication of photoemitted electrons in cylindrical alumina cells, containing cesium vapor, submitted to a longitudinal electric field and to intense laser pulses. We present several diagnoses allowing us to attribute this charge multiplication to efficient secondary electron emission (SEE) from the accelerated photoelectrons colliding with the inner wall at grazing incidence. Machining millimeter-size triangular grooves on the initially smooth inner wall, so as to prevent grazing incidence, is shown to be efficient in reducing SEE. The atomic signal characteristic of the space charge accumulated close to the anode is found to be reduced by more than one order of magnitude. This result is of important significance, not only for our parity-violation experiment in cesium vapor, but also for experiments and techniques involving SEE at grazing incidence. Laboratoire de l’Ecole Normale Supérieure associé au CNRS (UMR 8552) et à l’Université Pierre et Marie Curie. Received: 25 June 2002 / Revised version: 11 September 2002 / Published online: 15 November 2002 RID="*" ID="*"Present address: Institute for Physical Research, Ashtarak-2, 378 410 Armenia. RID="**" ID="**"Corresponding author. Fax: +33-1/4432-3434, E-mail:marianne@lkb.ens.fr     

Calculation of the temperature and thermal expansion of a STM tip heated by a short laser pulse

A mathematical model for the calculation of the temperature field in a scanning tunneling microscope (STM) tip under laser illumination is developed. The duration of the laser pulse is a few nanoseconds or shorter. A Gaussian distribution of the laser light intensity in time and space is assumed. Two different mechanisms of tip heating are taken into account: 1. due to an enhanced electric field on the tip; 2. due to heating of the side surface of the tip by the focused spot of laser light. An average tip temperature is calculated using the heat conductivity equation. The enhanced electric field on the tip is calculated by the method of boundary integral equations. Received: 20 August 2002 / Revised version: 4 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-2551/962-490, E-mail: sklein@fh-muenster.de     

Ion irradiation of exchange bias systems for magnetic sensor applications

It has been demonstrated that He⁺ ion irradiation is an excellent tool for modifying magnetic properties, like the magnetic anisotropy, the interlayer exchange coupling strength and the exchange bias field of ultra-thin magnetic layered systems. This paper summarizes the effects of ion irradiation on exchange bias systems. As a first example, for possible applications of the ion induced magnetic effects, the realization of an angle sensing device is described. Received: 11 November 2002 / Accepted: 12 November 2002 / Published online: 4 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-631-205-4095, E-mail: fassbend@physik.uni-kl.de RID="**" ID="**"Present address: Université de Rouen, Rouen, France     

A simple wet-chemical synthesis and characterization of CuO nanorods

Using a simple wet-chemical route, we synthesized CuO nanorods with diameters of ca. 5–15 nm and lengths of up to 400 nm. The purity, crystallinity, morphology, structure features, and chemical composition of the as-prepared CuO nanorods were investigated by powder X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Received: 22 March 2002 / Accepted: 12 June 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: wangqun@nju.edu.cn     

Investigation of the interaction of evaporated aluminum with vapor deposited Teflon AF films via X-ray photoelectron spectroscopy

The interfacial bonding and mixing between evaporated aluminum and a vapor deposited Teflon AF (abbreviated to AF) film have been investigated with X-ray photoelectron spectroscopy. Graphite carbon (C–C), and aluminum carbide (Al–C), oxide (Al–O–C) and fluoride (Al–F) are formed when aluminum atoms are deposited on to the AF film. With increasing deposition of aluminum, the concentrations of these newly formed components increase gradually. Moreover, in situ annealing results in remarkable increases in the C–C, Al–C, Al–O–C and Al–F configurations and a decrease in metallic aluminum. No significant diffusion of aluminum into the AF film was observed during the annealing. The Al compounds form a layer at the Al/AF interface that acts as an adhesion promoter and diffusion barrier. Received: 21 October 2002 / Accepted: 22 October 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +49-431/880-6229, E-mail: sjding@yahoo.com     

Study of beam-fanning hysteresis in photo- refractive SBN:Ce: light-induced and primary scattering as functions of polar structure

We study light-induced scattering (beam-fanning) in the photorefractive crystal SBN:Ce as a function of the polar structure of the crystal. The spatial structure of the beam-fanning is measured at different externally applied electric fields, and an optical hysteresis is found in the scattering. It is shown that the scattering hysteresis results from a polarization hysteresis typical for ferroelectrics in the polar phase. New information about primary scattering in SBN is obtained, and a corresponding model of its origin is proposed. It is shown that the intensity and angular distribution of the primary scattering strongly depend on the polar structure of the crystal and can be affected by the subsequent action of an external field and coherent illumination. Received: 27 August 2002 / Revised version: 19 December 2002 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. E-mail: mirco.imlau@uni-osnabrueck.de     

Buried interfaces studied by photoelectron diffraction

Angle- and energy-scanned photoelectron diffraction data can be used to investigate structures below surfaces. The modulations in photoelectron intensity result from diffraction of the emitted electron wave at neighbor atoms. In the past, scanned-energy photoelectron diffraction had been mainly used to determine the adsorption site of molecules at surfaces. Recent data show, however, that the technique can also be employed to obtain information about the upper substrate layer(s). At low kinetic energies, backward scattering is strong and in scanned-angle photoelectron diffraction the recorded patterns result from backward- and multiple-scattering effects. For a structural analysis, the intensity modulations have to be compared with the results for simulations performed for model clusters. As an example, recent angle-scanned photoelectron diffraction patterns recorded for the technologically important silicon oxide/silicon interface were compared with simulations. At the Si(001) surface orientation, the interface is extended over a few layers, whereas at the Si(111) surface orientation the transition is rather abrupt and occurs within one or two layers. Received: 9 September 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-231/755-3657, E-mail: carsten.westphal@physik.uni-dortmund.de     

Si<Subscript>x</Subscript>Ge<Subscript>1-x</Subscript> ultrahigh-vacuum chemical vapor deposition on Si(111)(7×7) from GeH<Subscript>4</Subscript>/Si<Subscript>2</Subscript>H<Subscript>6</Subscript> mixtures

We report the results of a study on ultrahigh-vacuum chemical vapor deposition of Si_xGe_1-x layers on Si(111)(7×7) with GeH₄ and Si₂H₆ mixtures. Using combined scanning tunneling microscopy and X-ray photoelectron spectroscopy, structural properties, the growth kinetics and the composition of the deposited alloys are analyzed as a function of the growth temperature for two different GeH₄:Si₂H₆ mixture ratios. The mutual influence of the precursors is shown by comparing the structures formed during deposition and the sticking coefficients of Si₂H₆ and GeH₄ with results obtained from exposure of Si(111) to the pure gases. Received: 28 July 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-731/502-5452, E-mail: hubert.rauscher@chemie.uni-ulm.de     

Contacting single bundles of carbon nanotubes with alternating electric fields

Single bundles of carbon nanotubes have been selectively deposited from suspensions onto sub-micron electrodes with alternating electric fields. We show that it is possible to control the trapping of a single bundle by the use of Ag as electrode material which, unlike Au, strongly interacts with the carboxyl functionalized carbon nanotubes. Excellent alignment of the bundles between Au or Ag electrodes occurs at frequencies above 1 kHz, with superior contacts being formed with Ag electrodes. Received: 22 May 2002 / Accepted: 21 June 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +49-7247/82-6368, E-mail: ralph.krupke@int.fzk.de     

Two-photon dissociation of HgBr2 as passive mode locking mechanism of a long-pulse XeCl excimer laser

Results are presented related to the use of two-photon dissociation of HgBr₂ molecular vapor as a non-linear fast-opening passive mode locker of a long-pulse XeCl laser. A dye solution, with or without scatterers, is also examined for comparison. Pulses with durations below our resolution limit of 400 ps and with a modulation better than 90% are reported. The low threshold, the non-destruction and the broadband absorption causing photodissociation make this molecule useful for laser pulse compression and decoupling for many lasers in the ultraviolet region. Received: 22 April 2002 / Revised version: 2 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +30-2810/394201, E-mail: efthim@physics.uoc.gr     

Micromagnetism of two-dimensional permalloy particles with different aspect ratios

Micromagnetic properties of the Fe₁₉Ni₈₁ (5 nm)/NiO (50 nm)/Fe₁₉Ni₈₁ (30 nm) structured system have been investigated in a photoemission electron microscope in the magnetic X-ray circular dichroism operating mode. The microstructured Fe₁₉Ni₈₁ (5 nm) film contained two-dimensional islands with the aspect ratio varying from 1:1 to 10:1, and the linear size of their long axis comprised 24, 12 and 6 μm. It is shown that the magnetic domains have the direction of magnetization preferentially parallel and antiparallel to the magnetic field direction in which this system was prepared. Their number is determined by the particles’ sizes, their shape as well as by the direction of the external magnetizing field and can be characterized by a non-monotonic size dependence. The magnetization of domains with different lateral sizes was found to be 0.4 T with an accuracy better than 20%. Received: 29 May 2002 / Accepted: 17 July 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-6131/392-3807, E-mail: nepijko@mail.uni-mainz.de     

Line width of an atom laser

We describe the measurement of the line width of an atom laser beam extracted from a Bose–Einstein condensate. Using a novel magnetic resonance imaging technique, we find that the energy width of the atom laser beam is Fourier-limited by the duration of the output-coupling process. Received: 25 July 2002 / Revised version: 28 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +41-1/633-1254, E-mail: koehl@iqe.phys.ethz.ch     

X-ray photoelectron spectroscopy study of magnetic films

Ta/NiO_x/Ni₈₁Fe₁₉/Ta films were prepared by rf reactive and dc magnetron sputtering. The exchange coupling field H_ex and the coercivity H_c of NiO_x/Ni₈₁Fe₁₉ as a function of the ratio of Ar to O₂ during the deposition process were studied. The composition and the chemical states in the interface region of NiO_x/NiFe were also investigated using X-ray photoelectron spectroscopy (XPS) and the peak decomposition technique. The results show that the ratio of Ar to O₂ has a great effect on the chemical states of nickel in NiO_x films. The exchange coupling field H_ex and the coercivity H_c of Ta/NiO_x/Ni₈₁Fe₁₉/Ta are thus seriously affected. XPS is shown to be a powerful tool for characterizing magnetic films. Received: 18 July 2001 / Accepted: 21 December 2001 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-010/6232-7283, E-mail: guanghua_yu@263.net     

Mode splitting in an optical slab waveguide filled with nematic liquid crystals

Mode splitting was observed when a He-Ne laser beam was reflected through a prism-coupled liquid-crystal slab waveguide under an applied electric field. The splitting manifests itself as the imposed voltage reaches a critical level, and diminishes when the voltage increases above a critical high value. If the applied voltage increases even further, mode splitting vanishes, attributing to the result that almost all the directors of the liquid crystal turn upright to the surface. The multi-layer matrix simulation can satisfactorily account for this phenomenon by exploiting the property of the anisotropic optical birefringence of a liquid crystal under applied voltages. Received: 2 December 2002 / Revised version: 24 January 2003 / Published online: 23 May 2003 RID="*" ID="*"Corresponding author. Fax: +886/3572-3052, E-mail: jtlue@phys.nthu.edu.tw     

Spectroscopy of cesium atoms adsorbing and desorbing at a dielectric surface

We use evanescent waves in a counterpropagating beams configuration to study the adsorption/desorption processes at a Cs vapor/dielectric interface in a sealed cell. Atoms close to the surface are velocity-selectively spin-polarized before adsorption by an amplitude-modulated pump beam. We subsequently observe the contribution of the desorbed atoms to the probe-beam absorption by way of phase-sensitive detection. We measure the number of desorbing polarized Cs atoms as a function of the surface temperature. The analysis of results is done through a simple thermodynamical model for the atomic desorption and we discuss its validity to infer the adsorption energy of the cesium atoms on a fused-quartz surface. Received: 15 May 2002 / Revised version: 7 August 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +55-83/216-7513, E-mail: martine@otica.ufpb.br     

Electrochemical synthesis of ordered CdTe nanowire arrays

Semiconductor CdTe nanowire arrays embedded in the nanochannels of the porous anodic alumina (PAA) template have been prepared by using a potentiostatic electrochemical deposition method. The morphology and structure of the CdTe nanowire arrays have been characterized by X-ray powder diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy. It is found that the CdTe nanowires with diameters and lengths of about 60 nm are single-crystalline with cubic phase structure, uniformly and continuously embedded in the nanochannels of the PAA template. X-ray energy-dispersion analysis and X-ray photoelectron spectroscopy analysis indicate that stoichiometric CdTe was formed. The growth mechanism of the CdTe nanowires is also discussed. Received: 11 June 2002 / Accepted: 2 July 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-551/559-1434, E-mail: aiwuzhao@yahoo.com.cn