arXiv subjects

P. Mueller

Publications and source records attributed to P. Mueller.

52 records · Page 3Linked to original sources

arXiv2004

Laser Spectroscopic Determination of the 6He Nuclear Charge Radius

We have performed precision laser spectroscopy on individual 6He (t1/2 = 0.8 s) atoms confined and cooled in a magneto-optical trap, and measured the isotope shift between 6He and 4He to be 43,194.772 +/- 0.056 MHz for the 2 3S1 - 3 3P2 transition. Based on this measurement and atomic theory, the nuclear charge radius of 6He is determined, for the first time in a method independent of nuclear models, to be 2.054 +/- 0.014 fm. The result is compared with the values predicted by a number of nuclear structure calculations, and tests their ability to characterize this loosely bound, halo nucleus.

nucl-ex↗

arXiv2004

Fine Structure of the 1s3p ^3P_J Level in Atomic ^4He: Theory and Experiment

We report on a theoretical calculation and a new experimental determination of the 1s3p ^3P_J fine structure intervals in atomic ^4He. The values from the theoretical calculation of 8113.730(6) MHz and 658.801(6) MHz for the nu_{01} and nu_{12} intervals, respectively, disagree significantly with previous experimental results. However, the new laser spectroscopic measurement reported here yields values of 8113.714(28) MHz and 658.810(18) MHz for these intervals. These results show an excellent agreement with the theoretical values and resolve the apparent discrepancy between theory and experiment.

physics.atom-ph↗

arXiv2004

One million year old groundwater in the Sahara revealed by krypton-81 and chlorine-36

Measurements of 81Kr/Kr in deep groundwater from the Nubian Aquifer (Egypt) were performed by a new laser-based atom-counting method. 81Kr ages range from \~2x10^5 to ~1x10^6 yr, correlate with 36Cl/Cl ratios, and are consistent with lateral flow of groundwater from a recharge area near the Uweinat Uplift in SW Egypt. Low delta-2H values of the 81Kr-dated groundwater reveal a recurrent Atlantic moisture source during Pleistocene pluvial periods. These results indicate that the 81Kr method for dating old groundwater is robust and such measurements can now be applied to a wide range of hydrologic problems.

physics.geo-ph↗

arXiv2004

Searches for Stable Strangelets in Ordinary Matter: Overview and a Recent Example

Our knowledge on the possible existence in nature of stable exotic particles depends solely upon experimental observation. Guided by this general principle and motivated by theoretical hypotheses on the existence of stable particles of strange quark matter, a variety of experimental searches have been performed. We provide an introduction to the theoretical hypotheses, an overview of the past searches, and a more detailed description of a recent search for helium-like strangelets in the Earth's atmosphere using a sensitive laser spectroscopy method.

nucl-ex↗

arXiv2003

Laser Spectroscopic Measurement of Helium Isotope Ratios

A sensitive laser spectroscopic method has been applied to the quantitative determination of the isotope ratio of helium at the level of 3He/4He = 10^-7 - 10^-5. The resonant absorption of 1083 nm laser light by the metastable 3He atoms in a discharge cell was measured with the frequency modulation saturation spectroscopy technique while the abundance of 4He was measured by a direct absorption technique. The results on three different samples extracted from the atmosphere and commercial helium gas were in good agreement with values obtained with mass spectrometry. The achieved 3-sigma detection limit of 3He in helium is 4 x 10^-9. This demonstration required a 200 micro-L STP sample of He. The sensitivity can be further improved, and the required sample size reduced, by several orders of magnitude with the addition of cavity enhanced spectroscopy.

physics.atom-ph↗

arXiv2003

A New Method of Measuring 81Kr and 85Kr Abundances in Environmental Samples

We demonstrate a new method for determining the 81Kr/Kr ratio in environmental samples based upon two measurements: the 85Kr/81Kr ratio measured by Atom Trap Trace Analysis (ATTA) and the 85Kr/Kr ratio measured by Low-Level Counting (LLC). This method can be used to determine the mean residence time of groundwater in the range of 10^5 - 10^6 a. It requires a sample of 100 micro-l STP of Kr extracted from approximately two tons of water. With modern atmospheric Kr samples, we demonstrate that the ratios measured by ATTA and LLC are directly proportional to each other within the measurement error of +/- 10%; we calibrate the 81Kr/Kr ratio of modern air measured using this method; and we show that the 81Kr/Kr ratios of samples extracted from air before and after the development of the nuclear industry are identical within the measurement error.

physics.atom-ph↗

arXiv2003

Counting individual 41Ca atoms with a magneto-optical trap

Atom Trap Trace Analysis (ATTA), a novel method based upon laser trapping and cooling, is used to count individual atoms of 41Ca present in biomedical samples with isotopic abundance levels between 10^-8 and 10^-10. ATTA is calibrated against Resonance Ionization Mass Spectrometry, demonstrating a good agreement between the two methods. The present ATTA system has a counting efficiency of 2x10^-7. Within one hour of observation time, its 3-sigma detection limit on the isotopic abundance of 41Ca reaches 4.5x10^-10.

physics.atom-ph↗

arXiv2003

Search for anomalously heavy isotopes of helium in the Earth's atmosphere

Motivated by the theoretical hypotheses on the existence of heretofore unobserved stable elementary particles and exotic nuclear states, we searched for doubly-charged particles, as anomalously heavy isotopes of helium, in the Earth's atmoshpere using a sensitive laser spectroscopy technique. The concentration of noble-gas-like atoms in the atmosphere and the subsequent very large depletion of the light He-3,4 isotopes allow us to set stringent limits on th abundance: 10^-13 to 10^-17 per atom in the solar system, over the mass range of 20 to 10,000 amu.

nucl-ex↗

arXiv2003

Ferromagnetic coupling and magnetic anisotropy in molecular Ni(II) squares

We investigated the magnetic properties of two isostructural Ni(II) metal complexes [Ni4Lb8] and [Ni4Lc8]. In each molecule the four Ni(II) centers form almost perfect regular squares. Magnetic coupling and anisotropy of single crystals were examined by magnetization measurements and in particular by high-field torque magnetometry at low temperatures. The data were analyzed in terms of an effective spin Hamiltonian appropriate for Ni(II) centers. For both compounds, we found a weak intramolecular ferromagnetic coupling of the four Ni(II) spins and sizable single-ion anisotropies of the easy-axis type. The coupling strengths are roughly identical for both compounds, whereas the zero-field-splitting parameters are significantly different. Possible reasons for this observation are discussed.

cond-mat.str-el↗

arXiv2001

Intrinsic Tunneling in Cuprates and Manganites

The most anisotropic high temperature superconductors like Bi2Sr2CaCu2O8, as well as the recently discovered layered manganite La1.4Sr1.6Mn2O7 are layered metallic systems where the interlayer current transport occurs via sequential tunneling of charge carriers. As a consequence, in Bi2Sr2CaCu2O8 adjacent CuO2 double layers form an intrinsic Josephson tunnel junction while in in La1.4Sr1.6Mn2O7 tunneling of spin polarized charge carriers between adjacent MnO2 layers leads to an intrinsic spin valve effect. We present and discuss interlayer transport experiments for both systems. To perform the experiments small sized mesa structures were patterned on top of single crystals of the above materials defining stacks of a small number of intrinsic Josephson junctions and intrinsic spin valves, respectively.

cond-mat.supr-con↗

arXiv2000

Phonons in intrinsic Josephson systems with parallel magnetic field

Subgap resonances in the I-V curves of layered superconductors are explained by the coupling between Josephson oscillations and phonons with dispersion in c-direction. In the presence of a magnetic field applied parallel to the layers additional structures due to fluxon motion appear. Their coupling with phonons is investigated theoretically and a shift of the phonon resonances in strong magnetic fields is predicted.

cond-mat.supr-con↗

arXiv2000

A 1400-MHz pilot search for young pulsars

We have used the Effelsberg 100-m radio telescope to conduct a 1400-MHz ($λ$ 21-cm) search for young and rapidly rotating radio pulsars along a 2 deg$^2$ strip of the northern Galactic plane defined by $28^{\circ} \leq l \leq 30^{\circ}$ and $|b|\leq0.5^{\circ}$. This region lies close to the Scutum spiral arm which is already known to contain a number of radio and X-ray pulsars. The search was nominally sensitive to pulsars with 1400-MHz flux densities above 0.3 mJy; this represents a threefold improvement in sensitivity over all previous searches of this region of the Galaxy. Four new long-period pulsars were discovered as a result of this survey. All three previously known pulsars in this region were also detected. The four new pulsars are relatively young ($<$ 0.5 Myr), weak ($<1$ mJy) sources with dispersion measures in the range 180--910 cm$^{-3}$ pc. None of the newly-discovered pulsars are associated with catalogued supernova remnants.

astro-ph↗

arXiv2000

The Effelsberg Search for Pulsars in the Galactic Centre

We report the status of a search for pulsars in the Galactic Centre, using a completely revised and improved high-sensitivity double-horn system at 4.85-GHz. We also present calculations about the success rate of periodicity searches for such a survey, showing that in contrast to conclusions in recent literature pulsars can be indeed detected at the chosen search frequency.

astro-ph↗

arXiv1997

A microscopic model for the intrinsic Josephson tunneling in high-T_C superconductors

A quantitative analysis of a microscopic model for the intrinsic Josephson effect in high-temperature superconductors based on interlayer tunneling is presented. The pairing correlations in the CuO_2-planes are modelled by a 2-D Hubbard-model with attractive interaction, a model which accounts well for some of the observed features such as the short planar coherence length. The stack of Hubbard planes is arranged on a torus, which is threaded by a magnetic flux. The current perpendicular to the planes is calculated as a function of applied flux (i. e. the phase), and -- after careful elimination of finite-size effects due to single-particle tunneling -- found to display a sinusoidal field dependence in accordance with interlayer Josephson tunneling. Studies of the temperature dependence of the supercurrent reveal at best a mild elevation of the Josephson transition temperature compared to the planar Kosterlitz-Thouless temperature. These and other results on the dependence of the model parameters are compared with a standard BCS evaluation.

cond-mat.str-el↗

arXiv1997

Coupling between phonons and intrinsic Josephson oscillations in cuprate superconductors

The recently reported subgap structures observed in the current-voltage characteristic of intrinsic Josephson junctions in the high-T_c superconductors Tl_2Ba_2Ca_2Cu_3O_{10+δ} and Bi_2Sr_2CaCu_2O_{8+δ} are explained by the coupling between c-axis phonons and Josephson oscillations. A model is developed where c-axis lattice vibrations between adjacent superconducting multilayers are excited by the Josephson oscillations in a resistive junction. The voltages of the lowest structures correspond well to the frequencies of longitudinal c-axis phonons with large oscillator strength in the two materials, providing a new measurement technique for this quantity.

cond-mat.supr-con↗

arXiv1995

The functional-analytic versus the functional-integral approach to quantum Hamiltonians: The one-dimensional hydrogen atom.

The capabilities of the functional-analytic and of the functional-integral approach for the construction of the Hamiltonian as a self-adjoint operator on Hilbert space are compared in the context of non-relativistic quantum mechanics. Differences are worked out by taking the one-dimensional hydrogen atom as an example, that is, a point mass on the Euclidean line subjected to the inverse-distance potential. This particular choice is made with the intent to clarify a long-lasting discussion about its spectral properties. In fact, for the four-parameter family of possible Hamiltonians the corresponding energy-dependent Green functions are derived in closed form. The multiplicity of Hamiltonians should be kept in mind when modelling certain experimental situations as, for instance, in quantum wires.

cond-mat↗