Metallic ground state and glassy transport in single crystalline URh2_2Ge2_2: Enhancement of disorder effects in a strongly correlated electron system

We present a detailed study of the electronic transport properties on a single crystalline specimen of the moderately disordered heavy fermion system URh$_2$Ge$_2$. For this material, we find glassy electronic transport in a single crystalline compound. We derive the temperature dependence of the electrical conductivity and establish metallicity by means of optical conductivity and Hall effect measurements. The overall behavior of the electronic transport properties closely resembles that of metallic glasses, with at low temperatures an additional minor spin disorder contribution. We argue that this glassy electronic behavior in a crystalline compound reflects the enhancement of disorder effects as consequence of strong electronic correlations.Comment: 5 pages, 4 figures, accepted for publication in PR

Giant spin canting in the S = 1/2 antiferromagnetic chain [CuPM(NO3)2(H2O)2]n observed by 13C-NMR

We present a combined experimental and theoretical study on copper pyrimidine dinitrate [CuPM(NO3)2(H2O)2]n, a one-dimensional S = 1/2 antiferromagnet with alternating local symmetry. From the local susceptibility measured by NMR at the three inequivalent carbon sites in the pyrimidine molecule we deduce a giant spin canting, i.e., an additional staggered magnetization perpendicular to the applied external field at low temperatures. The magnitude of the transverse magnetization, the spin canting of 52 degrees at 10 K and 9.3 T and its temperature dependence are in excellent agreement with exact diagonalization calculations.Comment: 5 pages, 6 Postscript figure

Quantum critical dynamics of a S = 1/2 antiferromagnetic Heisenberg chain studied by 13C-NMR spectroscopy

We present a 13C-NMR study of the magnetic field driven transition to complete polarization of the S=1/2 antiferromagnetic Heisenberg chain system copper pyrazine dinitrate Cu(C_4H_4N_2)(NO_3)_2 (CuPzN). The static local magnetization as well as the low-frequency spin dynamics, probed via the nuclear spin-lattice relaxation rate 1/T_1, were explored from the low to the high field limit and at temperatures from the quantum regime (k_B T << J) up to the classical regime (k_B T >> J). The experimental data show very good agreement with quantum Monte Carlo calculations over the complete range of parameters investigated. Close to the critical field, as derived from static experiments, a pronounced maximum in 1/T_1 is found which we interpret as the finite-temperature manifestation of a diverging density of zero-energy magnetic excitations at the field-driven quantum critical point.Comment: 5 pages, 4 figure

Spectroscopic study of the magnetic ground state of Nb1−y_{1-y}Fe2+y_{2+y}

We have investigated single crystals and polycrystals from the series Nb$_{1-y}$Fe$_{2+y}$, $-0.004 \leq y \leq 0.018$ by electron spin resonance, muon spin relaxation and M\"ossbauer spectroscopy. Our data establish that at lowest temperatures all samples exhibit bulk magnetic order. Slight Fe-excess induces low-moment ferromagnetism, consistent with bulk magnetometry ($\simeq 0.06 ~\mu_B/{\rm Fe}$), Nb--rich and stoichiometric NbFe$_2$ display spin density wave order with small magnetic moment amplitudes of the order $\sim 0.001 - 0.01 ~\mu_B/{\rm Fe}$. This provides microscopic evidence for a modulated magnetic state on the border of ferromagnetism in NbFe$_2$.Comment: 7 pages, 9 figure

Field-induced coupled superconductivity and spin density wave order in the Heavy Fermion compound CeCoIn5

The high field superconducting state in CeCoIn5 has been studied by transverse field muon spin rotation measurements with an applied field parallel to the crystallographic c-axis close to the upper critical field Hc2 = 4.97 T. At magnetic fields >= 4.8 T the muon Knight shift is enhanced and the superconducting transition changes from second order towards first order as predicted for Pauli-limited superconductors. The field and temperature dependence of the transverse muon spin relaxation rate sigma reveal paramagnetic spin fluctuations in the field regime from 2 T < H < 4.8 T. In the normal state close to Hc2 correlated spin fluctuations as described by the self consistent renormalization theory are observed. The results support the formation of a mode-coupled superconducting and antiferromagnetically ordered phase in CeCoIn5 for H directed parallel to the c-axis.Comment: 5 paes, 4 figure

Magnetoelastic and structural properties of azurite Cu3(CO3)2(OH)2 from neutron scattering and muon spin rotation

Azurite, Cu3(CO3)2(OH)2, has been considered an ideal example of a one-dimensional (1D) diamond chain antiferromagnet. Early studies of this material imply the presence of an ordered antiferromagnetic phase below $T_N \sim 1.9$ K while magnetization measurements have revealed a 1/3 magnetization plateau. Until now, no corroborating neutron scattering results have been published to confirm the ordered magnetic moment structure. We present recent neutron diffraction results which reveal the presence of commensurate magnetic order in azurite which coexists with significant magnetoelastic strain. The latter of these effects may indicate the presence of spin frustration in zero applied magnetic field. Muon spin rotation, $\mu$SR, reveals an onset of short-range order below 3K and confirms long-range order below $T_N$.Comment: 5 pages, 4 figures, PHYSICAL REVIEW B 81, 140406(R) (2010

Antiferromagnetic Dimers of Ni(II) in the S=1 Spin-Ladder Na_2Ni_2(C_2O_4)_3(H_2O)_2

We report the synthesis, crystal structure and magnetic properties of the S=1 2-leg spin-ladder compound Na_2Ni_2(C_2O_4)_3(H_2O)_2. The magnetic properties were examined by magnetic susceptibility and pulsed high field magnetization measurements. The magnetic excitations have been measured in high field high frequency ESR. Although the Ni(II) ions form structurally a 2-leg ladder, an isolated dimer model consistently describes the observations very well. The analysis of the temperature dependent magnetization data leads to a magnetic exchange constant of J=43 K along the rungs of the ladder and an average value of the g-factor of 2.25. From the ESR measurements, we determined the single ion anisotropy to D=11.5 K. The validity of the isolated dimer model is supported by Quantum Monte Carlo calculations, performed for several ratios of interdimer and intradimer magnetic exchange and taking into account the experimentally determined single ion anisotropy. The results can be understood in terms of the different coordination and superexchange angles of the oxalate ligands along the rungs and legs of the 2-leg spin ladder.Comment: 8 pages, 10 figure