Charge-carrier density collapse in La0.67Ca0.33MnO3and La0.67Sr0.33MnO3 epitaxial thin films

We measured the temperature dependence of the linear high field Hall resistivity of ( K) and ( K) thin films in the temperature range from 4 K up to 360 K in magnetic fields up to 20 T. At low temperatures we find a charge-carrier density of 1.3 and 1.4 holes per unit cell for the Ca- and Sr-doped compound, respectively. In this temperature range electron-magnon scattering contributes to the longitudinal resistivity. At the ferromagnetic transition temperature a dramatic drop in the number of charge-carriers n down to 0.6 holes per unit cell, accompanied by an increase in unit cell volume, is observed. Corrections of the Hall data due to a non saturated magnetic state will lead a more pronounced charge-carrier density collapse. Received 22 July 1999 and Received in final form 7 October 1999     

Contributions to photodoping in oxygen-deficient YBa2Cu3Ox films

We report our studies on the superconducting and normal-state properties of metallic thin films ( 52 K) exposed to long-term white-light illumination (photodoping). It was observed that the effects of photoexcitation strongly depended on the temperature at which the photodoping was performed. At low temperatures, both the Hall mobility and the Hall number were photoenhanced, whereas, at temperatures slightly below room temperature, the Hall mobility initially showed an abrupt increase followed by a long-term decrease, and the Hall number increased even stronger than at low temperatures. The enhancement of the film's superconducting transition temperature T_c, caused by photodoping, exhibited the same temperature dependence as the enhancement of the Hall number, being largest ( 2.6 K) at high temperatures. From the asynchronous behavior of the Hall quantities, we conclude that both the photoassisted oxygen ordering and charge transfer mechanisms contribute to photodoping. The relative contributions of both mechanisms and, thus, the electronic properties of the photoexcited state are strongly temperature dependent. Studies of the relaxation of the photoexcited state at 290 K showed an unexpectedly short relaxation time of the Hall mobility after termination of the illumination. The relaxation saturated somewhat below the initial, undoped value, similarly to the decrease of the Hall mobility, observed upon long illumination. These latter findings give evidence for a competition between the oxygen ordering and thermal disordering processes during and after the photoexcitation in the high-temperature range. Received: 13 October 1997 / Accepted: 19 November 1997     

Doppler limited laser spectroscopy on hafnium lines. Part II: Hyperfine structure of odd-parity levels

The hyperfine structure of selected odd-parity levels of the configurations 5d6s ^26p and 5d ^26s6p of I was studied in 10 lines lying in the red spectral region. Hyperfine spectra were obtained by the method of laser induced fluorescence in the plasma of a liquid nitrogen cooled hollow cathode discharge. The observed hyperfine structure constants A and B, together with results from earlier studies were analyzed by means of a parametric method. The interpretation has been carried out based on a refined multiconfigurational fine structure calculation including the main Rydberg series configurations mutually interacting. The set of fine structure parameters as well as the leading eigenvector percentages of levels relevant for this paper are given. The following single electron hfs parameters were deduced for : , ,, for the lowest configuration. Received: 16 November 1998 / Received in final form: 5 February 1999     

Induced ferromagnetism and colossal magnetoresistance by Ir-doping in Pr 1 - x Ca x MnO 3

The doping of the manganese site by iridium (up to 15%) in the small A cation manganites Pr_1-xCa_xMnO₃ ( 0.4 ? x ? 0.8), has been investigated as a new method to suppress charge-ordering and induce CMR effects. Ir doping leads to ferromagnetism and to insulator to metal transitions, with high transition temperatures reaching 180 K and CMR ratio in 7 T as large as 10⁴. The efficiency with which iridium induces ferromagnetism and CMR is compared to previous results obtained with other substitutions (Ru, Rh, Ni, Cr...). The ionic radius of the foreign cations and their mixed-valencies are found to be the main parameters governing the ability to collapse the charge-ordered state. Received 14 May 2001 and Received in final form 2 July 2001     

Photo-induced increase of the superconducting coherence lengths in oxygen-deficient YBa2Cu3Ox thin films

() thin films were photodoped with white light at various temperatures from 70 K to 290 K. Before and after the excitation, the magnetoconductivity was measured in a magnetic field B = 0.5 T, and the experimental results were fitted to the Aslamazov-Larkin theory of superconducting order-parameter fluctuations to determine the superconducting coherence lengths, and . We observed that the photodoping process enhanced and and that the amount increased with the photodoping temperature increase. On the other hand, the superconducting anisotropy / decreased with increasing temperature. The photodoping effect enhances superconducting properties of partially oxygen-deficient samples and is considerably increased by high doping temperatures. Received 15 December 1999 and Received in final form 24 May 2000     

Structural features of the La-Sr-Fe-Co-O system

A structural study has been performed on the La_0.8Sr_0.2Fe_xCo_1-xO₃ (x = 0.025 to 0.3) system displaying large magnetoresistance (MR) at room temperature. A detailed analysis of the crystal structure and microstructure was done by X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM). The atomic resolution TEM images and the appearing superreflections in the corresponding SAED patterns revealed that a superstructure is formed due to the presence of iron. The correlation between the ordered microstructure and the observed large MR ratio is discussed. ⁵⁷Fe M?ssbauer spectroscopy was utilized to gain information on the valence state of iron in the sample with x = 0.3. The lattice parameters of Fe- doped La_0.8Sr_0.2Fe_xCo_1-xO₃ compounds were found to increase monotonously with increasing Fe content. The valence state of iron was found to be Fe³⁺. Received 22 January 2001     

Extraction of hyperfine anomalies without precise values of the nuclear magnetic dipole moment

A new method for extracting the hyperfine anomaly from experimental hyperfine structure constants is suggested. Instead of independent high-precision measurements of the nuclear magnetic dipole moment, precise measurements of magnetic dipole hyperfine interaction constants for two atomic states and a theoretical analysis can be used. This can lead to determination of hyperfine anomaly for radioactive isotopes where the nuclear magnetic dipole moment is not known with high accuracy. Received: 17 February 1998     

Mechanism of grain-boundary magnetoresistance in Fe O films

The magnetotransport properties of magnetite films with different microstructures were investigated in order to identify prerequisites for the attainment of a large tunnelling magnetoresistance in polycrystalline samples. Epitaxial films on MgAl₂O₄, polycrystalline films on Al₂O₃ and rough MgAl₂O₄ substrates and a polycrystalline La_0.7Ca_0.3MnO₃ film on MgO were compared. Although grain boundaries induce a large high-field magnetoresistance in magnetite films, the low-field magnetoresistance characteristic for spin-polarized tunnelling was virtually absent in these samples. Two factors might be responsible for this behaviour: (1) grain boundaries in magnetite are conducting and do not form tunnelling barriers and (2) the spin-polarization near grain boundaries is suppressed due to non-stoichiometry. Received 15 April 2002 Published online 13 August 2002     

Isotope shift studies in the first spectrum of dysprosium: confirmation of assignments to and new assignments to configuration

Isotope shifts (-) are reported for 221 spectral lines of the neutral dysprosium atom (Dy I) in the region 470-565 nm using a photoelectric recording Fabry-Perot spectrometer and highly enriched isotopic samples of and excited in liquid-nitrogen-cooled hollow-cathode lamps. Isotope shift data for 173 of these lines are being reported for the first time. Using the isotope shift data, term isotope shifts (-) have been evaluated for 99 even- and 68 odd-parity energy levels of Dy I. New values have been obtained for 24 odd and 36 even levels. These new values have enabled us to check some of the existing tentative assignments and also to suggest configuration assignments to a few unassigned energy levels. The earlier tentative assignments of 4 f10 5 d 6 p configuration to many high odd levels lying above have been presently confirmed. We could assign the 4f^95d6s6p configuration to 24 unassigned even levels. Received: 3 August 1998     

Hyperfine structure in the atomic spectrum of niobium

We report on hyperfine structure measurements in 21 lines of atomic niobium in the spectral region from 640 nm to 870 nm by means of optogalvanic laser spectroscopy and laser induced fluorescence spectroscopy using a hollow cathode discharge and a tunable single-mode cw ring laser. Hyperfine structure constants A and B of altogether 29 excited energy levels were determined, 18 of them for the first time. Received 18 July 2002 / Received in final form 8 November 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: sk@kalium.physik.tu-berlin.de     

The influence of the dynamics of ionic multiplets onto electronic transport properties of heavy-fermion systems: a semi-phenomenological approach

We present calculations of the electronic transport properties of heavy-fermion systems within a semi-phenomenological approach to the dynamical mean field theory. In this approach the dynamics of the Hund's rules 4f (5f )-ionic multiplet split in a crystalline environment is taken into account. Within the scope of this calculation we use the linear response theory to reproduce qualitative features of the temperature-dependent resistivity and hall conductivity, the magneto-resistivity and the thermoelectric power typical for heavy-fermion systems. The model calculations are directly compared with experimental results on CeCu ₂ Si ₂. Received 30 June 2000 and Received in final form 15 December 2000     

Interpretation of the magnetoresistance in doped magnetic tunnel junctions

We present a quantum mechanical model of the magnetoresistance in ferromagnetic tunnel junctions artificially doped by the introduction of layers of impurities in the middle of the barrier. The electron transport across the barrier is described by a combination of direct tunneling, tunneling assisted by spin-conserving scattering and tunneling assisted by spin-flip scattering. With this model, we interpret recent experimental results concerning the dependence of the TMR amplitude on the amount of impurities in the barrier and on temperature. Received 1st February 2001 and Received in final form 14 June 2001     

Nuclear size correction to the hyperfine splitting in low- hydrogen-like atoms

The finite nuclear size effect on the hyperfine splitting of low-Z hydrogen-like atoms is studied in the external field approximation. A simple non-relativistic formula is proposed which expresses the nuclear size correction to the hyperfine splitting in terms of moments of the nuclear charge and magnetization distribution. The numerical results obtained via this formula are compared with corresponding results derived by means of the Zemach formula. A relativistic formula for the nuclear size correction to the hyperfine splitting is also derived. Received 18 March 2002 / Received in final form 14 November 2002 Published online 28 January 2003 RID="a" ID="a"e-mail: plunien@physik.tu-dresden.de     

Magnetic moments for lithium quartet S states

The fully correlated calculations of the Zeeman g_J factors for the first three quartet S states of lithium are presented, including relativistic and radiative corrections of orders α², α² m/M, and α³. The isotope shifts in g_J are predicted precisely for various isotopes of lithium. Received 4 December 2000 and Received in final form 26 September 2001     

Evolution of ( La 1 - y Pr y ) 0.7 Ca 0.3 MnO 3 crystal structure with A-cation size, temperature, and oxygen isotope substitution

The atomic structure of ( La _{1 - y} Pr _y ) _0.7 Ca _0.3 MnO ₃ compound with 0.5≤ y ≤1 has been systematically studied by neutron powder diffraction in the temperature range from 15 to 293 K. For composition with y = 0.75, the structural analysis was performed on two samples, one containing the natural mixture of oxygen isotopes and the other one 75% enriched by ¹⁸ O. The room temperature structural characteristics of the series, including cell volume, average Mn-O bond distance, and average Mn-O-Mn bond angle, are the linear functions of the < r _A >. Temperature dependencies of these parameters are quite smooth, except for the point T = T _FM , where a jump like changes occur. The isotope enriched samples have been found identical in crystal and magnetic structure down to the temperature of transition of the sample with ¹⁶ O into the metallic ferromagnetic phase. It confirms that different transport and magnetic properties of the samples with ¹⁶ O and ¹⁸ O at low temperature are driven by the different oxygen atoms dynamics solely. Temperature dependencies of the CO and AFM diffraction peak intensities and of the peak widths for compositions close to the metal-insulator boundary ( y ≈ 0.75) indicate the macroscopically phase separated AFM-dielectric + FM-metallic state below T _FM . Received 28 April 2000     

Jahn-Teller, charge and magnetic ordering in half-doped manganese oxides

The phase diagram of half-doped manganite systems of formula A _0.5 A ^′ _0.5MnO₃ is investigated within a single-orbital model incorporating magnetic double-exchange and superexchange, together with intersite Coulomb and electron-lattice interactions. Strong Jahn-Teller and breathing mode deformations compete together and result in shear lattice deformations. The latter stabilize the charge-ordered CE-type phase, which undergo first-order transitions with temperature or magnetic field to either Ferromagnetic metallic or Paramagnetic insulating phases. An essential feature is the self-consistent screening of Coulomb and electron-phonon interactions in the ferromagnetic phase. Received 28 November 2000     

Charge radius change in the heavy tin isotopes until A = 132 from laser spectroscopy

Laser spectroscopy measurements have been carried out on the very neutron-rich tin isotopes with the COMPLIS experimental setup. Using the 5s ²5p ²³ P ₀ → 5s ²5p6s ³ P ₁ optical transition, hyperfine spectra of ^126-132Sn and ^{125m, 127m, 129m-131m}Sn where recorded for the first time. The variation of the mean-square charge radius ( δ〈r ²〉) between these nuclei and nuclear moments of the isomers and the odd isotopes were thus measured. An odd-even staggering which inverts at A = 130 is clearly observed. This indicates a small appearance of a plateau on the δ〈r ²〉 which has to be confirmed by measuring the isotope shift beyond A = 132. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: leblanc@ipno.in2p3.fr     

Influence of high-energy electron irradiation on the transport properties of La1-xCaxMnO3 films ($$(x \approx 1/3)$$ )

The effect of crystal lattice disorder on the conductivity and colossal magnetoresistance in La_1-xCa_xMnO₃ ( ) films has been examined. The lattice defects are introduced by irradiating the film with high-energy ( MeV) electrons with a maximal fluence of about cm^-2. This comparatively low dose of irradiation produces rather small radiation damage in the films. The number of displacements per atom (dpa) in the irradiated sample is about 10^-5. Nevertheless, this results in an appreciable increase in the film resistivity. The percentage of the resistivity increase in the ferromagnetic metallic state (below the Curie temperature ) was much greater than that observed in the insulating state (above ). At the same time irradiation has much less effect on or on the magnitude of the colossal magnetoresistance. A possible explanation of such behavior is proposed. Received 21 July 1999 and Received in final form 27 December 1999     

Oxygen deficiency dependence of magnetic and magnetoresistive properties for La0.5Sr0.5CoO3-x thin films prepared by pulsed laser deposition

The complete (001)-oriented thin films of La_0.5Sr_0.5CoO_3-x (LSCO) are deposited on (001) SrTiO₃ substrates by pulsed laser deposition under reduced oxygen pressure. It is revealed that the c axis of the film stretches with depleting oxygen. The magnetic, electrical, and magnetoresistive properties of the films are characterized by means of various techniques. Significant dependence of these properties on oxygen deficiency in the films is demonstrated, with enhanced magnetoresistance recorded for the samples deposited over a wide range of reduced oxygen pressure. Received: 9 July 1998 / Accepted: 15 January 1999 / Published online: 31 March 1999     

Hyperfine structure and isotope shift measurements on 4d10 1S0 → 4d9 5p J=1 transitions in Pd I using deep-UV cw laser spectroscopy

The 4 d ^{10 1} S₀ ground-state transitions to the 4 d ⁹ 5 p configuration of palladium (Pd) have been studied. For this purpose, a tunable, single-mode, deep-UV cw laser has been built to generate the sum frequency of a frequency-doubled Ti:S laser with a second Ti:S laser. The produced wavelengths range from 244 to 276 nm. From the measured spectra the frequency splitting due to hyperfine structure and isotope shift, the hyperfine structure A and B constants and the lifetimes of the states have been extracted. Received 3 October 2001