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

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

Static and Dynamic Behaviour of Gas Bubbles in T-Shaped Non-Clogging Micro-Channels

Abstract Preventing micro-channels from clogging is a major issue in most micro and nanofluidic systems (Gravesen et al., J Micromech Microeng 3(4):168-182, 1993; Jensen et al., In: Proc. of MicroTAS 2002, Nara, Japan, pp 733-735, 2002 Wong et al., J Fluid Mech 292:71-94, 1995). The T-shaped channel first reported by Kohnle et al. (In: IEEE MEMS, the 15th international IEEE micro electro mechanical conference (ed), Las Vegas, pp 77-80, 2002) prevents micro-channels from clogging by the aid of the equilibrium bubble position in such a geometry. This work is concerned with the static and dynamic behaviour of bubbles in such T-shaped microchannels. The aspect ratio of a rectangle enclosing the Tshaped channel and the contact angle of the walls are the main parameters influencing the static and dynamic bubble behaviour. It is investigated in this article how these parameters relate to the equilibrium bubble shape and how optimum bubble velocities can be achieved inside the channel. An analytical model depending on the contact angle and the channel geometry is presented that allows to determine the bubble configuration inside the channel by minimizing the bubble&apos;s surface energy. A second model is derived to predict the velocity of gas bubbles driven by buoyancy in vertical T-shaped channels. The model is applied to design T-shaped channels with a maximum mobility of gas bubbles. Experiments with MEMS fabricated devices and CFD simulations are used to verify the models. Furthermore design rules for an optimum nonclogging channel geometry which provides the highest gas bubble mobility are given

Tuning the S=1/2S = 1/2 square-lattice antiferromagnet Sr2_2Cu(Te1−x_{1-x}Wx_x)O6_6 from N\'eel order to quantum disorder to columnar order

The spin-1/2 square-lattice Heisenberg model is predicted to have a quantum disordered ground state when magnetic frustration is maximized by competing nearest-neighbor $J_1$ and next-nearest-neighbor $J_2$ interactions ($J_2/J_1 \approx 0.5$). The double perovskites Sr$_2$CuTeO$_6$ and Sr$_2$CuWO$_6$ are isostructural spin-1/2 square-lattice antiferromagnets with N\'eel ($J_1$ dominates) and columnar ($J_2$ dominates) magnetic order, respectively. Here we characterize the full isostructural solid solution series Sr$_2$Cu(Te$_{1-x}$W$_x$)O$_6$ ($0 \leq x \leq 1$) tunable from N\'eel order to quantum disorder to columnar order. A spin-liquid-like ground state was previously observed for the $x$ = 0.5 phase, but we show that the magnetic order is suppressed below 1.5 K in a much wider region of $x \approx$ 0.1-0.6. This coincides with significant $T$-linear terms in the low-temperature specific heat. However, density functional theory calculations predict most of the materials are not in the highly frustrated $J_2/J_1 \approx 0.5$ region square-lattice Heisenberg model. Thus, a combination of both magnetic frustration and quenched disorder is the likely origin of the spin-liquid-like state in $x$ = 0.5.Comment: 20+5 pages, 6+4 figures. Accepted for publication in PR

Thin Film, Near-Surface and Multi-Layer Investigations by Low-Energy μ +SR

At the Paul Scherrer Institute (PSI, Villigen, Switzerland) the beam of low-energy positive polarised muons (LE-μ +) with tunable energy between 0.5 and 30keV allows the extension of the muon-spin-rotation technique (μSR) to studies on thin films and multi-layers (LE-μ +SR). The range of these muons in solids covers the near-surface region up to implantation depths of about 300nm. As a sensitive local magnetic probe with a complementary observational time window to other techniques LE-μ +SR offers the unique possibility to gain new insights in these nano-scale objects. After outlining the current status of the LE-μ + beam line we demonstrate the potential of this new technique by presenting the results of recent experiments: i) the direct observation of non-local effects in a superconducting Pb film, ii) the oxygen isotope effect on the in-plane penetration depth in optimally doped $${\text{YBa}}_{2} {\text{Cu}}_{3} {\text{O}}_{{7 - \delta }}$$ , and iii) the first observation of the conduction electron spin polarisation in the Ag spacer of a Fe/Ag/Fe tri-laye

Structure and magnetic order in Fe2+xV1-xAl

We present a detailed structural investigation via neutron diffraction of differently heat treated samples Fe2VAl and Fe2+xV1-xAl. Moreover, the magnetic behaviour of these materials is studied by means of mSR and Mossbauer-experiments. Our structural investigation indicates that quenched Fe2VAl, exhibiting the previously reported "Kondo insulating like" behaviour, is off-stoichiometric (6%) in its Al content. Slowly cooled Fe2VAl is structurally better ordered and stoichiometric, and the microscopic magnetic probes establish long range ferromagnetic order below TC = 13K, consistent with results from bulk experiments. The magnetic state can be modelled as being generated by diluted magnetic ions in a non-magnetic matrix. Quantitatively, the required number of magnetic ions is too large as to be explained by a model of Fe/V site exchange. We discuss the implications of our findings for the ground state properties of Fe2VAl, in particular with respect to the role of crystallographic disorder.Comment: accepted for publication in J. Phys.: Condens. Matte