Temperature and field dependence of the magnetic properties of Nd \mathsf{_{2-x}}Ce\mathsf{_{x}}CuO\mathsf{_4}

We have investigated the magnetic ordering of Nd_2-x Ce _x CuO₄ for x = 0, 0.09, 0.13, 0.15 and 0.18 by neutron diffraction at low temperatures down to 33 mK and under magnetic field up to 5 tesla applied along [1,-1,0] crystallographic direction. At zero applied magnetic field Cu magnetic sublattice orders at , 210, 130 and 105 K for x = 0, 0.09, 0.13, 0.15, respectively. No long range magnetic order of the Cu could be detected for x = 0.18. The magnetic order of Nd was found in all samples, with a gradual increase of the polarized magnetic moment with decreasing temperature, saturating around 1 K. Hyperfine induced nuclear polarization of the Nd nuclear spin has been observed below about 400 mK for samples with x = 0, 0.13, 0.15 and 0.18. Field variation of the intensities of the principal and superstructure reflections of Nd_2-x Ce _x CuO₄ at millikelvin temperatures shows a field-induced second-order double-k to single-k phase transition at H _c = 0.75 and 0.56 tesla for samples with x = 0 and 0.15, respectively at T = 50 mK. We have also investigated the polarization of the Nd electronic sublattice due to the field of the Cu sublattice by the element specific X-ray resonant magnetic scattering investigation with synchrotron radiation.Received: 9 April 2004, Published online: 14 December 2004PACS: 75.25. + z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.)T. Chatterji: Has changed his surname from Chattopadhyay to Chatterji.     

Interplay between structural and magnetic properties of Lu\mathsf{_{2}}Fe \mathsf{_{17}} at high pressure

A crystal structure of Lu₂Fe₁₇ was studied by means of neutron powder diffraction method in the pressure range up to 1.65 GPa and ambient temperature. Modification of the magnetic properties corresponding to the pressure-induced change of the crystal structure of Lu₂Fe₁₇ is analysed on the basis of local moment model in the mean field approximation. The experimentally observed suppression of FM state and stabilization of the non-collinear AFM state under high pressure in Lu₂Fe₁₇ can be explained qualitatively from model calculations. FM and non-collinear AFM states in Lu₂Fe₁₇ are found to be nearly degenerate at ambient pressure with the estimated difference between the total energy minima of mRyd and the application of high pressure removes this degeneracy in the favor of non-collinear AFM state.Received: 6 September 2004, Published online: 5 November 2004PACS: 61.50.Ks Crystallographic aspects of phase transformations; pressure effects - 75.25. + z Spin arrangements in magnetically ordered materials - 61.12.Ld Neutron diffraction     

Crystal structure and magnetic ordering of RNi Si compounds

The element distributions and the magnetic ordering behaviour of compounds RNi₁₀Si₂ (R = Tb, Dy, Ho, Er, Tm) have been studied by neutron powder diffraction down to temperatures of 1.6 K. The compounds crystallize in an ordered variant of the ThMn₁₂ structure type in the tetragonal space group P4/nmm. An ordered 1:1 distribution of Ni and Si on sites 4d and 4e, respectively, corresponds to a modulation vector [0, 0, 1] with respect to the space group I4/mmm of the ThMn₁₂ structure. TbNi₁₀Si₂ orders antiferromagnetically below T _N = 4.5 K with a magnetic propagation vector of [0, 0, 1/2]. The magnetic Tb moments, 8.97(2) /Tb atom at 1.6 K, are aligned along the c-axis. The Ni sites in TbNi₁₀Si₂ do not carry any ordered magnetic moments. The compounds with R = Dy, Ho, Er, and Tm are paramagnetic down to 1.6 K and 3.0 K, respectively. Received 10 July 2002 / Received in final form 12 September 2002 Published online 29 October 2002     

Microwave fluctuational conductivity in YBa\mathsf{_{2}}Cu\mathsf{_{3}}O \mathsf{_{7-\delta}}

We present measurements of the microwave complex conductivity at 23.9 and 48.2 GHz in YBa₂Cu₃O $_{7-\delta}$ films, in the fluctuational region above T _c . With increasing temperature, the fluctuational excess conductivity drops much faster than the well-known calculations within the time-dependent Ginzburg-Landau theory [H. Schmidt, Z. Phys. 216, 336 (1968)]. Approaching the transition temperature, slowing down of the fluctuations is observed. We discuss the results in terms of a modified Gaussian theory for finite-frequency fluctuational conductivity, where renormalization is introduced in order to account for the $T\rightarrow T_{c}$ regime, and a spectral cutoff is inserted in order to discard high-momentum modes. The data are in excellent agreement with the modified theory, when formulated for three-dimensional, anisotropic superconductors, in the whole experimentally accessible temperature range, and for both frequencies.     

Structural studies of La \mathsf{_{1-x}}Sr\mathsf{_{x}}MnO \mathsf {_{3 + \delta}} (x = 0.1-1.0)

We report here for the first time (particularly for $x \ge 0.5$ ) a systematic structural study using Rietveld Profile Refinement of powder X-ray diffraction data on the series of polycrystalline compounds La_1-x Sr _x MnO $_{3 + \delta}$ (0.1 ≤ x ≤ 1.0). The iodometric redox titration results show that the compounds $0.1\le x\le 0.4$ and the end compound are oxygen excess and deficient respectively and the compounds in the compositional range 0.5 ≤ x ≤ 0.9 are oxygen stoichiometric within the experimental error. It is found that the structure remains hexagonal until x = 0.4 composition. On further doping, at x = 0.5 composition, a structural transition to orthorhombic phase is observed. Around this composition, very small variations in the Mn-O(2)-Mn and average Mn-Mn bond distances are observed. For above x = 0.5, until x = 0.8 composition, the structure remains orthorhombic with reduced orthorhombic distortion. For the next compound, x = 0.9, a mixed hexagonal and orthorhombic phase is observed where the hexagonal phase is 6 layered with stacking sequence of ABCACB type and the orthorhombic phase is more distorted than that of x = 0.8 composition. The end compound is a four layered hexagonal structure with stacking sequence ABAC type which is more distorted than ABCACB type. As one goes down the series, a decrease in the volume per formula unit and average Mn-O bond distance are observed except at x = 0.9 composition. The observed structural transitions from hexagonal to orthorhombic to layered hexagonal phase can be explained under the electrostatic limit.     

Feshbach resonances in $\mathsf{^{23}}$Na- $\mathsf{^{87}}$Rb

We describe s-wave collisions between ultracold ²³Na and ⁸⁷Rb atoms in the presence of a magnetic field. For any collision input channel knowledge of the singlet and triplet intermolecular potentials [1] enables us to predict the variation in scattering lengths as a function of the magnetic field. We employ the Born-Oppenheimer approximation to predict several Feshbach resonances for ²³Na⁸⁷Rb. We also include a discussion of why these resonances are expected to be narrow as observed in the case of ⁸⁷Rb₂.Received: 6 August 2004, Published online: 23 November 2004PACS: 34.90. + q Other topics in atomic collisions - 03.75.Hh Multicomponent condensates     

On the diffraction pattern of C \mathsf{_{60}} peapods

We present detailed calculations of the diffraction pattern of a powder of bundles of C₆₀ peapods. The influence of all pertinent structural parameters of the bundles on the diffraction diagram is discussed, which should lead to a better interpretation of X-ray and neutron diffraction diagrams. We illustrate our formalism for X-ray scattering experiments performed on peapod samples synthesized from 2 different technics, which present different structural parameters. We propose and test different criteria to solve the difficult problem of the filling rate determination.Received: 19 May 2004, Published online: 26 November 2004PACS: 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 61.10.Dp Theories of diffraction and scattering - 61.10.Nz X-ray diffraction     

Magnetism and hyperfine interactions in EuM2Ge2 and GdM2Ge2(M = Mn,Fe, Co,Ni, Cu)

X-ray, magnetic susceptibility and ¹⁵¹Eu, ¹⁵⁵Gd Mössbauer effect studies of EuM₂Ge₂ and GdM₂Ge₂ were performed. All compounds crystallize in the ThCr₂Si₂ body centered tetragonal structure. In all compounds, except those with M = Mn and in EuM₂Ge₂, the M component carries no magnetic moment. All compounds except those with Mn are antiferromagnetic at low temperatures. In EuMn₂Ge₂ the Mn moments order ferromagnetically at 330 K and change to antiferromagnetic order when the Eu moments order ferromagnetically (9 K). This behaviour is different from that in GdMn₂Ge₂, where the Mn sublattice orders antiferromagnetically at 365 K and becomes ferromagnetic and antiparallel to the ferromagnetic Gd sublattice at 96 K. The Mössbauer studies of ¹⁵¹Eu and ¹⁵¹Gd provide values for the magnetic hyperfine fields, the quadrupole interactions and the orientation of the magnetic moments relative to the local fourfold axis (c-axis). It turns out that in the Eu compounds the easy axis of magnetization is close to the c-axis, while in the Gd compounds it is in the basal plane. In all systems, excluding those with Mn, the interatomic rare earth-rare earth distances have the dominant effect on the conduction electron charge density and polarization at the rare earth site and on the Curie point.     

Insulating surface layer on single crystal K $\mathsf{_{3}}\mathsf{C}\mathsf{_{60}}$

Using angle-dependent photoemission spectra of core and valence levels we show that metallic, single crystal K³C₆₀ is terminated by an insulating or weakly-conducting surface layer. We attribute this to the effects of strong intermolecular correlations combined with the average surface charge state. Several controversies on the electronic structure are thereby resolved.Received: 16 July 2004, Published online: 5 November 2004PACS: 71.20.Tx Fullerenes and related materials; intercalation compounds - 73.20.-r Electron states at surfaces and interfaces - 79.60.Bm Clean metal, semiconductor, and insulator surfaces     

A non-magnetic Fe probe of multiple magnetic phase transitions in RMn2Si2−x Ge x,R=rare earth

Summary Dilute⁵⁷Fe M?ssbauer-spectroscopy studies of RMn₂X₂ (X=Si and/or Ge, R=La, Ce, Pr, Nd, Sm, Eu and Gd) at 4.2 to 650 K yield the following results: Fe in RMn₂X₂ does not carry a magnetic moment. It reveals the magnetic order in the Mn and R sublattices through transferred hyperfine fields (∼100 kOe). The compounds LaMn₂Si₂, LaMn₂Ge₂, CeMn₂Ge₂, PrMn₂Ge₂, NdMn₂Ge₂ and SmMn₂Ge₂, known to be ferromagnets withT _c=300–350 K, are antiferromagnetically ordered above their correspondingT _c. TherT _N values extend from 385 K (SmMn₂Ge₂) to 470 K (LaMn₂Si₂). At the ferromagnetic-antiferromagnetic phase transition, a sharp reorientation of the Mn magnetic moments relative to the crystalline axes occurs. In RMn₂Si_2−x Ge _x with intermediatex values, the Ge is much more dominant in determining the magnetic properties of the Mn sublattice. While PrMn₂Si₂ is an antiferromagnet (T _N=365 K) and PrMn₂Ge₂ is a ferromagnetantiferromagnet (T _c=328 K,T _N=415 K), we find that in PrMn₂SiGe, the magnetic behaviour is similar to that in pure PrMn₂Ge₂,T _c=305 K andT _N=395 K. Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

Magnetic ordering in HoRu2Si2, HoRh2Si2, TbRh2Si2 and TbIr2Si2 by neutron diffraction

Neutron diffraction study of polycrystalline HoRu₂Si₂, HoRh₂Si₂, TbRh₂Si₂, and TbIr₂Si₂ was performed in the temperature range between 4.2 and 300 K. For HoRu₂Si₂ the magnetic spin alignment of a linear transverse wave mode below the Néel temperature 19 K is observed. This static moment wave is propagating along the b-axis with k=(0, 0.2, 0) and is polarized in the c-axis. The root-mean-square and maximum saturation moments per Ho atom are 9.26 and 13.09μ_B, respectively. HoRh₂Si₂, TbRh₂Si₂ an TbIr₂Si₂ are simple collinear antiferromagnets of +-+- type with Néel temperatures of (27±1), (98±2) and (72±3) K, respectively. For TbRh₂Si₂ and TbIr₂Si₂ magnetic moments are localized on RE ions only and are aligned along the tetragonal axis, while for HoRh₂Si₂ they form an angle ø = (28±3)°.     

Magnetic properties of RAu2Si2 rare earth compounds

The magnetic properties of the R Au₂Si₂ compounds with R = Ce-Er have been investigated. It was found that the compounds for which R = Ce, Sm, Gd, Tb and Dy are antiferromagnetically ordered at temperatures ranging from 5.7 to 15.9°K. PrAu₂Si₂ and NdAu₂Si₂ exhibit paramagnetic behavior for temperatures as low as 4.2°K. The magnetic structure is ferrimagnetic for the compounds in which R = Eu, Ho, and Er. The Eu compound is in the divalent state. The Néel and Curie points for this system do not follow the De-Gemnes function. Curie-Weiss Behavior is exhibited by all the compounds with effective moments in good agreement with that of a free tripositive lanthanide ion. The difference in magnetic properties between R Au₂Si₂ and the isomorphous R Fe₂Si₂ series is discussed.     

A theoretical study of the Y$\mathsf{_{3}}$O clusters

Hybrid density functional calculations are performed to study the structural and electronic properties of neutral, anionic and cationic Y₃O clusters. The most stable structures of these clusters are found to be triply bridging oxygen atom structures with C_S symmetry. The ground states of Y₃O, Y₃O^- and Y₃O ⁺ are doublet (²A), triplet (³A) and singlet (¹A), respectively. The calculated electron affinities and ionization potentials are in good agreement with the available experimental data. Time-dependent density functional theory is used to calculate the low-lying excited states. A theoretical assignment for the features in the experimental photoelectron spectra is given.Received: 5 November 2003, Published online: 20 January 2004PACS: 36.40.Mr Spectroscopy and geometrical structure of clusters - 31.15.Ew Density-functional theory - 34.50.Gb Electronic excitation and ionization of molecules; intermediate molecular states (including lifetimes, state mixing, etc.)     

Rb$\mathsf{_{2}}$ diffuse band emission excited by diode lasers

We investigated collisional processes involved in the population of the Rb₂ diffuse band through resonant excitation of Rb atoms. Near-infrared (780 nm) and violet (420 nm) diode lasers were used for the Rb first ( 5 ²S_1/2? 5 ²P_3/2\rm 5 ^{2}S_{1/2}\to 5 ^{2}P_{3/2} ) and second ( 5 ²S_1/2? 6 ²P_3/2\rm 5 ^{2}S_{1/2}\to 6 ^{2}P_{3/2} ) resonant doublet excitations. Laser induced fluorescence spectra were detected and investigated at different rubidium densities, buffer gas pressures and excitation wavelengths.     

Precursor diamagnetism above the superconducting transition in YNi_\mathsf{2}B_\mathsf{2}C

High-resolution SQUID magnetization measurements in a single crystal of YNi₂B₂C around the superconducting transition are reported. The diamagnetic magnetization -M _fl at constant field H as a function of temperature and isothermal magnetization curves -M _fl vs. H are used to derive insights on precursor phenomena approaching the bulk transition temperature K. The precursor diamagnetism is found strongly enhanced with respect to the conventional Ginzburg-Landau value for Gaussian fluctuations and the curves -M _fl vs. H exhibit an upturn with the field and hysteretic effects up to T ^* = 15.4 K. These results are interpreted in terms of a non-zero order parameter in superconducting droplets above the bulk T _c . These droplets are likely to be related to inhomogeneities resulting from small amount of boron to carbon substitutions.Received: 23 April 2003, Published online: 15 October 2003PACS: 74.40.+k Fluctuations (noise, chaos, nonequilibrium superconductivity, localization, etc.) - 74.20.De Phenomenological theories (two-fluid, Ginzburg-Landau, etc.) - 74.25.Ha Magnetic properties     

Crystals and magnetic structure of RMn2Si2 (R = Pr, Nd, Y) and YMn2Ge2

Crystallographic and magnetic properties of PrMn₂Si₂, NdMn₂Si₂, YMn₂Si₂ and YMn₂Ge₂ intermetallics were studied by X-ray, neutron diffraction and magnetometric measurements. The crystal structure of all four compounds was confirmed to be body-centered tetragonal (space group I4/mmm). All were found to be antiferromagnetic with Néel points at 368, 380, 460 and 395 K respectively. Neutron diffraction results indicate that their magnetic structure consists of ferromagnetic layers composed of Mn ions piled up along the c-axis. Each layer is antiferromagnetically coupled to adjacent layer. The magnetic space group is I_p4/m′m′m′. No magnetic ordering of the R sublattice was observed at 1.8 K in the case of R = Pr and Nd.     

On the low-temperature properties of TmCo2Ge2

In this paper low-temperature magnetic properties of TmCo₂Ge₂ are reported. The compound was investigated by means of magnetic, electrical transport, heat capacity and neutron diffraction measurements. Clear anomalies were found in all of the bulk characteristics at the onset of antiferomagnetically ordered state below 2.4 K. According to neutron diffraction data, Tm magnetic moments are aligned along the c-axis, forming the AFI-type magnetic structure. In the paramagnetic state, the specific heat reveals a distinct Schottky contribution with the total crystal field splitting of about 150 K.     

Electrical resistivity in the ferromagnetic metallic state of La-Ca-MnO\mathsf{_{3}}: Role of electron-phonon interaction

The temperature-dependent resistivity of the perovskite manganites La_1-x Ca _x MnO₃, with x = 0.33, is theoretically analysed within the framework of the classical electron-phonon model of resistivity, i.e., the Bloch-Gruneisen model. Due to inherent acoustic (low-frequency) phonons ( as well as high-frequency optical phonons ( , the contributions to the resistivity have first been estimated. The acoustic phonons of the oxygen-breathing mode yield a relatively larger contribution to the resistivity compared to the contribution of optical phonons. Furthermore, the nature of phonons changes around T = 167 K exhibiting a crossover from an acoustic to optical phonon regime with elevated temperature. The contribution to resistivity estimated by considering both phonons, i.e. and , when subtracted from thin film data, infers a power temperature dependence over most of the temperature range. The quadratic temperature dependence of is understood in terms of electron-electron scattering. Moreover, in the higher temperature limit, the difference can be varies linearly with T ^4.5 in accordance with the electron-magnon scattering in the double exchange process. Within the proposed scheme, the present numerical analysis of temperature dependent resistivity shows similar results as those revealed by experiment.Received: 8 April 2004, Published online: 12 August 2004PACS: 5.47.Gk Colossal magnetoresistance - 72.15.-v Electronic conduction in metals and alloys - 74.25.Kc Phonons - 75.30.Ds Spin wavesD. Varshney: dvboson.sop@dauniv.ac.in     

Magnetic properties of RCr2Si2 compounds (R=Tb,Er)

We report on magnetic properties of RCr₂Si₂ compounds with R=Tb, Er. The polycrystalline samples were characterized by powder X-ray diffraction and found to be isostructurally crystallizing in the ThCr₂Si₂-type tetragonal structure. The samples were further investigated by specific heat, AC-susceptibility and magnetization methods in the temperature range 2–900 K and in magnetic fields up to 9 T. The magnetic measurements revealed the magnetic ordering of the rare-earth moments below about 2 K, whilst no high-temperature ordering of the Cr moments was observed. The evidence of for Cr magnetism is corroborated by results of first-principles calculations based on the density functional theory (DFT).     

Surface second-harmonic generation in Au: TiO$\mathsf{_{2}}$ cermet thin films

The Surface Second-Harmonic Generation (SSHG) in Au: TiO₂ thin films of nanocermets is studied with samples of different metallic volume fractions using an IR Nd: YAG pulsed laser. We worked in a reflection geometry by varying the incidence angle and the directions of the incident beam polarization and of an analyzer of the Second Harmonic beam. We show that the SH signal is not due to electric dipolar processes only but can be treated within the general a, b, and d formalism where surfaces and bulk SH currents are well distinguished, and by describing the dielectric function of cermets within the effective-medium theory. The effective values obtained for a, b, and d are attributed to gold but the rather small discrepancies between them and those given by models are understandable by the fact that our samples are heterogeneous and not semi-infinite.Received: 19 March 2004, Published online: 6 July 2004PACS: 42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation - 78.66.-w Optical properties of specific thin films - 78.30.Ly Disordered solids