Effect of Ni-doping on magnetism and superconductivity in Eu0.5K0.5Fe2As2

The effect of Ni-doping on the magnetism and superconductivity in Eu0.5K0.5Fe2As2 has been studied through a systematic investigation of magnetic and superconducting properties of Eu0.5K0.5(Fe1-xNix)2As2 (x = 0, 0.03, 0.05, 0.08 and 0.12) compounds by means of dc and ac magnetic susceptibilities, electrical resistivity and specific heat measurements. Eu0.5K0.5Fe2As2 is known to exhibit superconductivity with superconducting transition temperature Tc as high as 33 K. The Ni-doping leads to a rapid decrease in Tc; Tc is reduced to 23 K with 3% Ni-doping, and 8% Ni-doping suppresses the superconductivity to below 1.8 K. In 3% Ni-doped sample Eu0.5K0.5(Fe0.97Ni0.03)2As2 superconductivity coexists with short range ordering of Eu2+ magnetic moments at Tm ~ 6 K. The suppression of superconductivity with Ni-doping is accompanied with the emergence of a long range antiferromagnetic ordering with TN = 8.5 K and 7 K for Eu0.5K0.5(Fe0.92Ni0.08)2As2 and Eu0.5K0.5(Fe0.88Ni0.12)2As2, respectively. The temperature and field dependent magnetic measurements for x = 0.08 and 0.12 samples reflect the possibility of a helical magnetic ordering of Eu2 moments. We suspect that the helimagnetism of Eu spins could be responsible for the destruction of superconductivity as has been observed in Co-doped EuFe2As2. The most striking feature seen in the resistivity data for x = 0.08 is the reappearance of the anomaly presumably due to spin density wave transition at around 60 K. This could be attributed to the compensation of holes (K-doping at Eu-site) by the electrons (Ni-doping at Fe site). The anomaly associated with spin density wave further shifts to 200 K for x = 0.12 for which the electron doping has almost compensated the holes in the system.Comment: 9 pages, 10 figure

Magnetic Transition in the Kondo Lattice System CeRhSn2

Our resistivity, magnetoresistance, magnetization and specific heat data provide unambiguous evidence that CeRhSn2 is a Kondo lattice system which undergoes magnetic transition below 4 K.Comment: 3 pages text and 5 figure

H - T phase diagram of YbCo2Si2 with H // [100]

We report on the first high-resolution dc-magnetisation ($M$) measurements on a single crystal of \ycs. $M$ was measured down to 0.05 K and in fields up to 12 T, with the magnetic field $H$ parallel to the crystallographic direction [100]. Two antiferromagnetic (AFM) phase transitions have been detected in a field $\mu_{0}H = 0.1$ T at $T_{N} = 1.75$ K and $T_{L} = 0.9$ K, in form of a sharp cusp and a sudden drop in $\chi = M/H$, respectively. These signatures suggest that the phase transitions are $2^{nd}$ order at $T_{N}$ and $1^{st}$ order at $T_{L}$. The upper transition is suppressed by a critical field $\mu_{0}H_{N} = 1.9$ T. The field-dependent magnetisation shows two hysteretic metamagnetic-like steps at the lowest temperature, followed by a sharp kink, which separates the AFM region from the paramagnetic one. The magnetic $H - T$ phase diagram of \ycs has been deduced from the isothermal and isofield curves. Four AFM regions were identified which are separated by $1^{st}$ and $2^{nd}$ order phase-transition lines.Comment: 5 Pages, 3 figure

High Resolution Photoemission Study on Low-T_K Ce Systems: Kondo Resonance, Crystal Field Structures, and their Temperature Dependence

We present a high-resolution photoemission study on the strongly correlated Ce-compounds CeCu_6, CeCu_2Si_2, CeRu_2Si_2, CeNi_2Ge_2, and CeSi_2. Using a normalization procedure based on a division by the Fermi-Dirac distribution we get access to the spectral density of states up to an energy of 5 k_BT above the Fermi energy E_F. Thus we can resolve the Kondo resonance and the crystal field (CF) fine-structure for different temperatures above and around the Kondo temperature T_K. The CF peaks are identified with multiple Kondo resonances within the multiorbital Anderson impurity model. Our theoretical 4f spectra, calculated from an extended non-crossing approximation (NCA), describe consistently the observed photoemission features and their temperature dependence. By fitting the NCA spectra to the experimental data and extrapolating the former to low temperatures, T_K can be extracted quantitatively. The resulting values for T_K and the crystal field energies are in excellent agreement with the results from bulk sensitive measurements, e.g. inelastic neutron scattering.Comment: 16 two-column pages, 10 figure

Superconductivity in SrFe_(2-x)Co_xAs_2: Internal Doping of the Iron Arsenide Layers

In the electron doped compounds SrFe_(2-x)Co_xAs_2 superconductivity with T_c up to 20 K is observed for 0.2 < x < 0.4. Results of structure determination, magnetic susceptibility, electrical resistivity, and specific heat are reported. The observation of bulk superconductivity in all thermodynamic properties -- despite strong disorder in the Fe-As layer -- favors an itinerant picture in contrast to the cuprates and renders a p- or d-wave scenario unlikely. DFT calculations find that the substitution of Fe by Co (x > 0.3) leads to the suppression of the magnetic ordering present in SrFe_2As_2 due to a rigid down-shift of the Fe-3d_(x^2-y^2) related band edge in the density of states.Comment: 5 pages, 3 figure

Investigation of the Spin-Peierls transition in CuGeO_3 by Raman scattering

Raman experiments on the spin-Peierls compound CuGeO$_3$ and the substituted (Cu$_{1- x}$,Zn$_x$)GeO$_3$ and Cu(Ge$_{1-x}$,Ga$_x$)O$_3$ compounds were performed in order to investigate the response of specific magnetic excitations of the one-dimensional spin-1/2 chain to spin anisotropies and substitution-induced disorder. In pure CuGeO$_3$, in addition to normal phonon scattering which is not affected at all by the spin-Peierls transition, four types of magnetic scattering features were observed. Below T$_{SP}$=14 K a singlet-triplet excitation at 30 cm$^{-1}$, two-magnon scattering from 30 to 227 cm$^{-1}$ and folded phonon modes at 369 and 819 cm$^{-1}$ were identified. They were assigned by their temperature dependence and lineshape. For temperatures between the spin-Peierls transition T$_{SP}$ and approximately 100 K a broad intensity maximum centered at 300 cm$^{-1}$ is observed.Comment: 7 pages, LaTex2e, including 3 figures (eps) to be published in Physica B (1996

Violation of critical universality at the antiferromagnetic phase transition of YbRh2Si2

We report on precise low-temperature specific-heat measurements, C(T), of YbRh2Si2 in the vicinity of the antiferromagnetic phase transition on a single crystal of superior quality (RRR 150). We observe a very sharp peak at T_N=72mK with absolute values as high as C/T=8J/molK^2. A detailed analysis of the critical exponent \alpha around T_N reveals \alpha=0.38 which differs significantly from those of the conventional universality classes in the Ginzburg-Landau theory, where \alpha<0.11. Thermal-expansion measurements corroborate this large positive critical exponent. These results provide insight into the nature of the critical magnetic fluctuations at a temperature-driven phase transition close to a quantum critical point.Comment: Accepted for PR

How chemical pressure affects the fundamental properties of rare-earth pnictides: an ARPES view

Angle-resolved photoelectron spectroscopy, supplemented by theoretical calculations has been applied to study the electronic structure of heavy-fermion material CeFePO, a homologue to the Fe-based high-temperature superconductors, and CeFeAs_0.7P_0.3O, where the applied chemical pressure results in a ferromagnetic order of the 4f moments. A comparative analysis reveals characteristic differences in the Fe-derived band structure for these materials, implying a rather different hybridization of valence electrons to the localized 4f orbitals. In particular, our results suggest that the ferromagnetism of Ce moments in CeFeAs_0.7P_0.3O is mediated mainly by Fe 3d_xz/yz orbitals, while the Kondo screening in CeFePO is instead due to a strong interaction of Fe 3d_3z^2-r^2 orbitals.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. B (Rapid

Magnetic phase diagram of YbCo2Si2 derived from magnetization measurements

We report on high-resolution dc-magnetization (M) measurements on a high-quality single crystal of YbCo2Si2. M was measured down to 0.05K and in fields up to 4T, with the magnetic field oriented along the crystallographic directions [100], [110] and [001] of the tetragonal structure