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     

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.     

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.     

Magnetic order in RCr\mathsf{_{2}}Si\mathsf{_{2}} intermetallics

The magnetic structure and ordering temperatures of three intermetallic compounds which crystallize in the tetragonal ThCr₂Si₂ structure, TbCr₂Si₂, HoCr₂Si₂ and ErCr₂Si₂, have been determined by neutron diffraction, differential scanning calorimetry and magnetization measurements. The Cr-sublattice orders anti-ferromagnetically with Néel temperatures of 758 K for TbCr₂Si₂, 718 K for HoCr₂Si₂ and 692 K for ErCr₂Si₂. Chromium atoms located at 4d crystallographic sites are aligned anti-parallel along the c-axis, with G_ZCr magnetic modes. In contrast with metallic bcc Cr, the refined room temperature value of the ordered Cr moment is anomalously large for all three compounds. No long range magnetic order of the R sublattice in TbCr₂Si₂ and HoCr₂Si₂ is observed, whilst the Er sublattice in ErCr₂Si₂ orders independently of the Cr sublattice below 2.4 K with moments ferromagnetically aligned in the basal plane.Received: 4 November 2003, Published online: 30 January 2004PACS: 75.25. + z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.) - 75.30.Cr Saturation moments and magnetic susceptibilities - 75.50.Ee Antiferromagnetics     

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