s-wave superconductivity in the noncentrosymmetric W3Al2C superconductor: an NMR study.

We report on a microscopic study of the noncentrosymmetric superconductor W3Al2C (withTc= 7.6 K), mostly by means of27Al- and13C nuclear magnetic resonance (NMR). Since in this material the density of states at the Fermi level is dominated by the tungsten's 5dorbitals, we expect a sizeable spin-orbit coupling (SOC) effect. The normal-state electronic properties of W3Al2C resemble those of a standard metal, but with a Korringa product 1/(T1T) significantly smaller than that of metallic Al, reflecting the marginal role played bys-electrons. In the superconducting state, we observe a reduction of the Knight shift and an exponential decrease of the NMR relaxation rate 1/T1, typical ofs-wave superconductivity (SC). This is further supported by the observation of a small but distinct coherence peak just belowTcin the13C NMR relaxation-rate, in agreement with the fully-gapped superconducting state inferred from the electronic specific-heat data well belowTc. The above features are compared to those of members of the same family, in particular, Mo3Al2C, often claimed to exhibit unconventional SC. We discuss why, despite the enhanced SOC, W3Al2C does not show spin-triplet features in its superconducting state and consider the broader consequences of our results for noncentrosymmetric superconductors in general.

Design optimization through thermomechanical finite-element analysis of a hybrid piston-clamped anvil cell for nuclear magnetic resonance experiments.

The investigation of materials under extreme pressure conditions requires high-performance cells whose design invariably involves trade-offs between the maximum achievable pressure, the allowed sample volume, and the possibility of real-time pressure monitoring. With a newly conceived hybrid piston-clamped anvil cell, we offer a relatively simple and versatile system, suitable for nuclear magnetic resonance experiments up to 4.4 GPa. Finite-element models, taking into account mechanical and thermal conditions, were used to optimize and validate the design prior to the realization of the device. Cell body and gaskets were made of beryllium-copper alloy and the pistons and pusher were made of tungsten carbide, while the anvils consist of zirconium dioxide. The low-temperature pressure cell performance was tested by monitoring in situ the pressure-dependent 63Cu nuclear-quadrupole-resonance signal of Cu2O.

High-T c superconductivity in undoped ThFeAsN.

Unlike the widely studied ReFeAsO series, the newly discovered iron-based superconductor ThFeAsN exhibits a remarkably high critical temperature of 30 K, without chemical doping or external pressure. Here we investigate in detail its magnetic and superconducting properties via muon-spin rotation/relaxation and nuclear magnetic resonance techniques and show that ThFeAsN exhibits strong magnetic fluctuations, suppressed below ~35 K, but no magnetic order. This contrasts strongly with the ReFeAsO series, where stoichiometric parent materials order antiferromagnetically and superconductivity appears only upon doping. The ThFeAsN case indicates that Fermi-surface modifications due to structural distortions and correlation effects are as important as doping in inducing superconductivity. The direct competition between antiferromagnetism and superconductivity, which in ThFeAsN (as in LiFeAs) occurs at already zero doping, may indicate a significant deviation of the s-wave superconducting gap in this compound from the standard s ± scenario.Exploring the interplay between the superconducting gap and the antiferromagnetic phase in Fe-based superconductors remains an open issue. Here, the authors show that Fermi-surface modifications by means of structural distortions and correlation effects are as important as doping in inducing superconductivity in undoped ThFeAsN.

A firmware-defined digital direct-sampling NMR spectrometer for condensed matter physics.

We report on the design and implementation of a new digital, broad-band nuclear magnetic resonance (NMR) spectrometer suitable for probing condensed matter. The spectrometer uses direct sampling in both transmission and reception. It relies on a single, commercially-available signal processing device with a user-accessible field-programmable gate array (FPGA). Its functions are defined exclusively by the FPGA firmware and the application software. Besides allowing for fast replication, flexibility, and extensibility, our software-based solution preserves the option to reuse the components for other projects. The device operates up to 400 MHz without, and up to 800 MHz with undersampling, respectively. Digital down-conversion with ±10 MHz passband is provided on the receiver side. The system supports high repetition rates and has virtually no intrinsic dead time. We describe briefly how the spectrometer integrates into the experimental setup and present test data which demonstrates that its performance is competitive with that of conventional designs.

1D to 2D Na+ ion diffusion inherently linked to structural transitions in Na0.7CoO2.

We report the observation of a stepwise "melting" of the low-temperature Na-vacancy order in the layered transition-metal oxide Na0.7CoO2. High-resolution neutron powder diffraction analysis indicates the existence of two first-order structural transitions, one at T1≈290 K followed by a second at T2≈400 K. Detailed analysis strongly suggests that both transitions are linked to changes in the Na mobility. Our data are consistent with a two-step disappearance of Na-vacancy order through the successive opening of first quasi-1D (T1>T>T2) and then 2D (T>T2) Na diffusion paths. These results shed new light on previous, seemingly incompatible, experimental interpretations regarding the relationship between Na-vacancy order and Na dynamics in this material. They also represent an important step towards the tuning of physical properties and the design of tailored functional materials through an improved control and understanding of ionic diffusion.

Field-induced quantum soliton lattice in a frustrated two-leg spin-1/2 ladder.

Based on high-field (31)P nuclear magnetic resonance experiments and accompanying numerical calculations, it is argued that in the frustrated S=1/2 ladder compound BiCu(2)PO(6) a field-induced soliton lattice develops above a critical field of µ(0)H(c1)=20.96(7) T. Solitons result from the fractionalization of the S=1, bosonlike triplet excitations, which in other quantum antiferromagnets are commonly known to experience Bose-Einstein condensation or to crystallize in a superstructure. Unlike in spin-Peierls systems, these field-induced quantum domain walls do not arise from a state with broken translational symmetry and are triggered exclusively by magnetic frustration. Our model predicts yet another second-order phase transition at H(c2)>H(c1), driven by soliton-soliton interactions, most likely corresponding to the one observed in recent magnetocaloric and other bulk measurements.

A two-axis goniometer for low-temperature nuclear magnetic resonance measurements on single crystals.

We report on the construction of a two-axis goniometer intended for low-temperature, single-crystal nuclear magnetic resonance (NMR) measurements. With the use of home-made and commercially available parts, our simple probe-head design achieves good sensitivity, while maintaining a high angular precision and the ability to orient samples also when cooled to liquid helium temperatures. The probe with the goniometer is adapted to be inserted into a commercial (4)He-flow cryostat, which fits into a wide-bore superconducting solenoid magnet. Selected examples of NMR measurements illustrate the operation of the device.

Distribution of NMR relaxations in a random Heisenberg chain.

NMR measurements of the (29)Si spin-lattice relaxation time T(1) were used to probe the spin-1/2 random Heisenberg chain compound BaCu(2)(Si(1-x)Ge(x))(2)O(7). Remarkable differences between the pure (x=0) and the fully random (x=0.5) cases are observed, indicating that randomness generates a distribution of local magnetic relaxations. This distribution, which is reflected in a stretched exponential NMR relaxation, exhibits a progressive broadening with decreasing temperature, caused by a growing inequivalence of magnetic sites. Compelling independent evidence for the influence of randomness is also obtained from magnetization data and Monte Carlo calculations. These results suggest the formation of random-singlet states in this class of materials, as previously predicted by theory.

Direct observation of impurity-induced magnetism in a spin-(1/2) antiferromagnetic Heisenberg two-leg spin ladder.

Nuclear magnetic resonance and magnetization measurements were used to probe the magnetic features of single-crystalline Bi(Cu(1-x)Zn(x))(2)PO(6) with 00 and we present clear evidence for a temperature-dependent variation of the local magnetization close to the Zn sites. The generic nature of this observation is indicated by results of model calculations on appropriate spin systems of limited size employing quantum Monte Carlo methods.

Melting of the Na layers in solid Na0.8CoO2.

Data of 23Na NMR spectra and relaxation measurements are interpreted as suggesting that, upon increasing temperature, the Na layers in Na0.8CoO2 adopt a 2D liquid state at T=291 K. The corresponding first order phase transition is preceded by a rapidly increasing mobility and diffusion of Na ions above 200 K. Above 291 K, the 23Na NMR response is similar to that previously observed in superionic conductors with planar Na layers.

Magneto-optical evidence of double exchange in a percolating lattice.

Substituting Eu by Ca in ferromagnetic EuB6 leads to a percolation limited magnetic ordering. We present and discuss magneto-optical data of the Eu(1-x)Ca(x)B6 series, based on measurements of the reflectivity R(omega) from the far infrared up to the ultraviolet, as a function of temperature and magnetic field. Via the Kramers-Kronig transformation of R(omega) we extract the complete absorption spectra of samples with different values of x. The change of the spectral weight in the Drude component by increasing the magnetic field agrees with a scenario based on the double-exchange model, and suggests a crossover from a ferromagnetic metal to a ferromagnetic Anderson insulator upon increasing Ca content at low temperatures.

Percolation and the colossal magnetoresistance of Eu-based hexaboride.

Upon substituting Ca for Eu in the local-moment ferromagnet EuB6, the Curie temperature T(C) decreases substantially with increasing dilution of the magnetic sublattice and is completely suppressed for x

Optical evidence for a spin-filter effect in the charge transport of Eu0.6Ca0.4B6.

We have measured the optical reflectivity R(omega) of Eu0.6Ca0.4B6 as a function of temperature (T) between 1.5 and 300 K and in external magnetic fields (H) up to 7 T. R(omega) increases with decreasing T and increasing H field, but the plasma edge feature does not exhibit the sharp onset and steep slope that is observed in EuB6. The analysis of the H-field dependence of the low-T optical conductivity confirms the previously observed exponential decrease of the electrical resistivity upon increasing bulk magnetization at constant T. The individual exponential magnetization dependences of the plasma frequency and scattering rate are also extracted from the optical data.

Unusual magnetic-field-induced phase transition in the mixed state of superconducting NbSe2.

The thermal conductivity kappa in the basal plane of single-crystalline hexagonal NbSe2 has been measured as a function of magnetic field H, oriented both along and perpendicular to the c axis, at several temperatures below T(c). With the magnetic field in the basal plane and oriented parallel to the heat flux we observed, in fields well below H(c2), an unexpected hysteretic behavior of kappa(H) with all the generic features of a first order phase transition. The transition is not manifest in the kappa(H) curves, if H is still in the basal plane but oriented perpendicularly to the heat-flux direction. The origin of the transition is not yet understood.

Low-dimensional spin S = 1/2 system at the quantum critical limit: Na2V3O7.

We report the results of measurements of the dc susceptibility and the 23Na-NMR response of Na2V3O7, a recently synthesized, nonmetallic low dimensional spin system. Our results indicate that, upon reducing the temperature to below 100 K, the V4+ moments are gradually quenched, leaving only one moment out of nine active. The NMR data reveal a phase transition at very low temperatures. With decreasing applied field H, the critical temperature shifts towards T=0 K, suggesting that Na2V3O7 may be regarded as an insulator reaching a quantum critical point at H=0.

High-temperature weak ferromagnetism in a low-density free-electron gas.

The magnetic properties of the ground state of a low-density free-electron gas in three dimensions have been the subject of theoretical speculation and controversy for seven decades. Not only is this a difficult theoretical problem to solve, it is also a problem which has not hitherto been directly addressed experimentally. Here we report measurements on electron-doped calcium hexaboride (CaB6) which, we argue, show that-at a density of 7× 1019 electrons cm-3-the ground state is ferromagnetically polarized with a saturation moment of 0.07 µB per electron. Surprisingly, the magnetic ordering temperature of this itinerant ferromagnet is 600 K, of the order of the Fermi temperature of the electron gas.