Crystal structure and phonon softening in Ca3Ir4Sn13

We investigated the crystal structure and lattice excitations of the ternary intermetallic stannide Ca3Ir4Sn13 using neutron and x-ray scattering techniques. For T > T* ~ 38 K the x-ray diffraction data can be satisfactorily refined using the space group Pm-3n. Below T* the crystal structure is modulated with a propagation vector of q = (1/2, 1/2, 0). This may arise from a merohedral twinning in which three tetragonal domains overlap to mimic a higher symmetry, or from a doubling of the cubic unit cell. Neutron diffraction and neutron spectroscopy results show that the structural transition at T* is of a second-order, and that it is well described by mean-field theory. Inelastic neutron scattering data point towards a displacive structural transition at T* arising from the softening of a low-energy phonon mode with an energy gap of Delta(120 K) = 1.05 meV. Using density functional theory the soft phonon mode is identified as a breathing mode of the Sn12 icosahedra and is consistent with the thermal ellipsoids of the Sn2 atoms found by single crystal diffraction data.

Distinct vortex-glass phases in Yb$_{3}$Rh$_{4}$Sn$_{13}$ at high and low magnetic fields

The vortex lattice (VL) in the mixed state of the stannide superconductor Yb$_{3}$Rh$_{4}$Sn$_{13}$ has been studied using small-angle neutron scattering (SANS). The field dependencies of the normalized longitudinal and transverse correlation lengths of the VL, $xi_L/a_0$ and $xi_T /a_0$, reveal two distinct anomalies that are associated with vortex-glass phases below $mu_0H_l$~$approx$~700~G and above $mu_{0}H_h$~$sim$~1.7~T ($a_0$ is the intervortex distance). At high fields, around 1.7~T, the longitudinal correlation decreases abruptly with increasing fields indicating a weakening (but not a complete destruction) of the VL due to a phase transition into a glassy phase, below $mu_{0}H_{c_2}$(1.8 K)~$approx$~2.5~T. $xi_L/a_0$ and $xi_T /a_0$, gradually decrease for decreasing fields of strengths less than 1~T and tend towards zero. The shear elastic modulus $c_{66}$ and the tilting elastic modulus $c_{44}$ vanish at a critical field $mu_0H_l$~$approx$~700~G, providing evidence for a disorder-induced transition into a vortex-glass. A ring of scattered intensity is observed for fields lower than 700~G, $i.e.$, $mu_{0}H_{c_1}$~=~135~G~$<$~$mu_{0}H$~$<$~700~G. This low-field phenomenon is of different nature than the one observed at high fields, where $xi_L/a_0$ but not $xi_T/a_0$, decreases abruptly to an intermediate value.

A neutron scattering study of the mixed state of Yb$_{3}$Rh$_{4}$Sn$_{13}$

Using the small angle neutron scattering (SANS) technique we investigated the vortex lattice (VL) in the mixed state of the stannide superconductor Yb$_{3}$Rh$_{4}$Sn$_{13}$. We find a single domain VL of slightly distorted hexagonal geometry for field strengths between 350 and 18500 G and temperatures between T = 0.05 and T = 6.5 K. We observe a clear in-plane rotation of the VL for different magnetic field directions relative to the crystallographic axes. We also find that the hexagonal symmetry of the VL is energetically favorable in Yb$_{3}$Rh$_{4}$Sn$_{13}$ for external fields oriented along axes of different symmetries: twofold [110], threefold [111] and fourfold [100]. The observed behavior is different from other conventional and unconventional superconductors. The superconducting state is characterized by an isotropic gapped order parameter with an amplitude of $Delta(0)$ = 1.57 $pm$ 0.05 meV. At the lowest temperatures the field dependence of the magnetic form factor in our material reveals a London penetration depth of $lambda_{L}$ = 2508 $pm$ 17 $AA$ and a Ginzburg coherence length of $xi$ = 100 $pm$ 1.3 $AA$, i.e., it is a strongly type-II superconductor, $kappa$ = $lambda_{L}/xi$ = 25.

1D to 2D Na Ion Diffusion Inherently Linked to Structural Transitions in Na$_{0.7}$CoO$_{2}$

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 indicates the existence of two first-order structural transitions, one at T1 = 290 K, followed by a second at T2 = 400 K. Detailed analysis reveals 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.

Direct observation of the quantum critical point in heavy fermion CeRhSi$_3$

We report on muon spin rotation studies of the noncentrosymmetric heavy fermion antiferromagnet CeRhSi$_3$. A drastic and monotonic suppression of the internal fields, at the lowest measured temperature, was observed upon an increase of external pressure. Our data suggest that the ordered moments are gradually quenched with increasing pressure, in a manner different from the pressure dependence of the Neel temperature. At $unit{23.6}{kbar}$, the ordered magnetic moments are fully suppressed via a second-order phase transition, and $T_{rm{N}}$ is zero. Thus, we directly observed the quantum critical point at $unit{23.6}{kbar}$ hidden inside the superconducting phase of CeRhSi$_3$.

New structural and magnetic aspects of the nanotube system Na2V3O7

We present new experimental results of low temperature x-ray synchrotron diffraction, neutron scattering and very low temperature (mK-range) bulk measurements on the nanotube system ${tube}$. The crystal structure determined from our data is similar to the previously proposed model (P. Millet {it et al.} J. Solid State Chem. $bf{147}$, 676 (1999)), but also deviates from it in significant details. The structure comprises nanotubes along the c-axis formed by stacking units of two V-rings buckled in the $ab$-plane. The space group is P$bar{3}$ and the composition is nonstoichiometric, Na(2-x)V3O7, x=0.17. The thermal evolution of the lattice parameters reveals anisotropic lattice compression on cooling. Neutron scattering experiments monitor a very weak magnetic signal at energies from -20 to 9 meV. New magnetic susceptibility, specific heat measurements and decay of remanent magnetization in the 30 mK - 300 mK range reveal that the previously observed transition at ~76 mK is spin-glass like with no long-range order. Presented experimental observations do not support models of isolated clusters, but are compatible with a model of odd-legged S=1/2 spin tubes possibly segmented into fragments with different lengths.

Pressure induced variation of the ground state of CeAl3

Pressure-induced variations of $^{27}$Al NMR spectra of CeAl$_3$ indicate significant changes in the ground-state characteristics of this prototypical heavy-electron compound. Previously reported magnetic and electronic inhomogeneities at ambient pressure and very low temperatures are removed with external pressures exceeding 1.2 kbar. The spectra and results of corresponding measurements of the NMR spin-lattice relaxation rates indicate a pressure-induced emergence of a simple paramagnetic state involving electrons with moderately enhanced masses and no magnetic order above 65 mK.

^{63,65}Cu NMR and NQR evidence for an unusual spin dynamics in PrCu_2 below 100 K

We report the results of a $^{63,65}$Cu NMR/NQR study probing the intermetallic compound PrCu$_2$. The previously claimed onset of magnetic order at 65 K, indicated in a $mu$SR study, is not confirmed. Based on our data we discuss different possible reasons for this apparent discrepancy, including a non negligible influence of the implanted muons on their environment. Competing dipolar and quadrupolar interactions lead to unusual features of the magnetic-ion/conduction-electron system, different from those of common intermetallics exhibiting structural or magnetic instabilities.