Evidence of a bond-nematic phase in LiCuVO4

Polarized and unpolarized neutron scattering experiments on the frustrated ferromagnetic spin-1/2 chain LiCuVO4 show that the phase transition at HQ of 8 Tesla is driven by quadrupolar fluctuations and that dipolar correlations are short-range with moments parallel to the applied magnetic field in the high-field phase. Heat-capacity measurements evidence a phase transition into this high-field phase, with an anomaly clearly different from that at low magnetic fields. Our experimental data are consistent with a picture where the ground state above HQ has a next-nearest neighbour bond-nematic order along the chains with a fluid-like coherence between weakly coupled chains.Comment: 5 pages, 4 figures. To appear in Phys. Rev. Let

Energy transfer processes in Er-doped SiO2 sensitized with Si nanocrystals

We present a high-resolution photoluminescence study of Er-doped SiO2 sensitized with Si nanocrystals (Si NCs). Emission bands originating from recombination of excitons confined in Si NCs and of internal transitions within the 4f-electron core of Er3+ ions, and a band centered at lambda = 1200nm have been identified. Their kinetics have been investigated in detail. Based on these measurements, we present a comprehensive model for energy transfer mechanisms responsible for light generation in this system. A unique picture of energy flow between subsystems of Er3+ and Si NCs is developed, yielding truly microscopic information on the sensitization effect and its limitations. In particular, we show that most of the Er3+ ions available in the system are participating in the energy exchange. The long standing problem of apparent loss of optical activity of majority of Er dopants upon sensitization with Si NCs is clarified and assigned to appearance of a very efficient energy exchange mechanism between Si NCs and Er3+ ions. Application potential of SiO2:Er sensitized by Si NCs is discussed in view of the newly acquired microscopic insight.Comment: 30 pages 13 figure

Extreme vortex pinning in the non-centrosymmetric superconductor CePt3_{3}Si

We report on the vortex dynamics of a single crystal of the non-centrosymmetric heavy-fermion superconductor CePt$_{3}$Si. Decays of the remnant magnetization display a clean logarithmic time dependence with rates that follow the temperature dependence expected from the Kim-Anderson theory. The creep rates are lower than observed in any other centrosymmetric superconductor and are not caused by high critical currents. On the contrary, the critical current in CePt$_{3}$Si is considerably lower than in other superconductors with strong vortex pinning indicating that an alternative impediment on the flux line motion might be at work in this superconductor.Comment: 4 pages, 5 figure

Anisotropic Exchange in LiCuVO4_4 probed by ESR

We investigated the paramagnetic resonance in single crystals of LiCuVO$_4$ with special attention to the angular variation of the absorption spectrum. To explain the large resonance linewidth of the order of 1 kOe, we analyzed the anisotropic exchange interaction in the chains of edge-sharing CuO$_6$ octahedra, taking into account the ring-exchange geometry of the nearest-neighbor coupling via two symmetric rectangular Cu-O bonds. The exchange parameters, which can be estimated from theoretical considerations, nicely agree with the parameters obtained from the angular dependence of the linewidth. The anisotropy of this magnetic ring exchange is found to be much larger than it is usually expected from conventional estimations which neglect the bonding geometry. Hence, the data yield the evidence that in copper oxides with edge-sharing structures the role of the orbital degrees of freedom is strongly enhanced. These findings establish LiCuVO$_4$ as one-dimensional compound at high temperatures. PACS: 76.30.-v, 76.30.Fc, 75.30.EtComment: 18 pages, 6 figure

Kondo Insulator to Semimetal Transformation Tuned by Spin-Orbit Coupling

Recent theoretical studies of topologically nontrivial electronic states in Kondo insulators have pointed to the importance of spin-orbit coupling (SOC) for stabilizing these states. However, systematic experimental studies that tune the SOC parameter $\lambda_{\rm{SOC}}$ in Kondo insulators remain elusive. The main reason is that variations of (chemical) pressure or doping strongly influence the Kondo coupling $J_{\text{K}}$ and the chemical potential $\mu$ -- both essential parameters determining the ground state of the material -- and thus possible $\lambda_{\rm{SOC}}$ tuning effects have remained unnoticed. Here we present the successful growth of the substitution series Ce$_3$Bi$_4$(Pt$_{1-x}$Pd$_x$)$_3$ ($0 \le x \le 1$) of the archetypal (noncentrosymmetric) Kondo insulator Ce$_3$Bi$_4$Pt$_3$. The Pt-Pd substitution is isostructural, isoelectronic, and isosize, and therefore likely to leave $J_{\text{K}}$ and $\mu$ essentially unchanged. By contrast, the large mass difference between the $5d$ element Pt and the $4d$ element Pd leads to a large difference in $\lambda_{\rm{SOC}}$, which thus is the dominating tuning parameter in the series. Surprisingly, with increasing $x$ (decreasing $\lambda_{\rm{SOC}}$), we observe a Kondo insulator to semimetal transition, demonstrating an unprecedented drastic influence of the SOC. The fully substituted end compound Ce$_3$Bi$_4$Pd$_3$ shows thermodynamic signatures of a recently predicted Weyl-Kondo semimetal.Comment: 6 pages, 5 figures plus Supplemental Materia

New high magnetic field phase of the frustrated S=1/2S=1/2 chain compound LiCuVO4_4

Magnetization of the frustrated $S=1/2$ chain compound LiCuVO$_4$, focusing on high magnetic field phases, is reported. Besides a spin-flop transition and the transition from a planar spiral to a spin modulated structure observed recently, an additional transition was observed just below the saturation field. This newly observed magnetic phase is considered as a spin nematic phase, which was predicted theoretically but was not observed experimentally. The critical fields of this phase and its dM/dH curve are in good agreement with calculations performed in a microscopic model (M. E. Zhitomirsky and H. Tsunetsugu, preprint, arXiv:1003.4096v2).Comment: 5 pages, 4 figure

Ion counting efficiencies at the IGISOL facility

At the IGISOL-JYFLTRAP facility, fission mass yields can be studied at high precision. Fission fragments from a U target are passing through a Ni foil and entering a gas filled chamber. The collected fragments are guided through a mass separator to a Penning trap where their masses are identified. This simulation work focuses on how different fission fragment properties (mass, charge and energy) affect the stopping efficiency in the gas cell. In addition, different experimental parameters are varied (e. g. U and Ni thickness and He gas pressure) to study their impact on the stopping efficiency. The simulations were performed using the Geant4 package and the SRIM code. The main results suggest a small variation in the stopping efficiency as a function of mass, charge and kinetic energy. It is predicted that heavy fragments are stopped about 9% less efficiently than the light fragments. However it was found that the properties of the U, Ni and the He gas influences this behavior. Hence it could be possible to optimize the efficiency.Comment: 52 pages, 44 figure