Superconductivity in Ce- and U-based "122" heavy-fermion compounds

This review discusses the heavy-fermion superconductivity in Ce- and U-based compounds crystallizing in the body-centered tetragonal ThCr2Si2 structure. Special attention will be paid to the theoretical background of these systems which are located close to a magnetic instability.Comment: 12 pages, 9 figures. Invited topical review (special issue on "Recent Developments in Superconductivity") Metadata and references update

Avalanche criticality in the martensitic transition of Cu67.64Zn16.71Al15.65 shape-memory alloy: a calorimetric and acoustic emission study

The first-order diffusionless structural transition in Cu67.64Zn16.71Al15.65 is characterized by jerky propagation of phase fronts related to the appearance of avalanches. In this paper, we describe a full analysis of this avalanche behavior using calorimetric heat-flux measurements and acoustic emission measurements. Two different propagation modes, namely, smooth front propagation and jerky avalanches, were observed in extremely slow measurements with heating and cooling rates as low as a few 10−3 K/h. Avalanches show criticality where each avalanche leads to a spike in the heat flux. Their statistical analysis leads to a power law [P(E)∼E−ε, where P(E)dE is the probability to observe an avalanche with energy E in an interval between E and E+dE] with an energy exponent of ε=2.15±0.15 in excellent agreement with the results of acoustic emission measurements. Avalanches appear to be more common for heating rates faster than 5×10−3 K/h whereas smooth front propagation occurs in all calorimetric measurements and (almost) exclusively for slower heating rates. Repeated cooling runs were taken after a waiting time of 1 month (and an intermediate heating run). Correlations between the avalanche sequences of the two cooling runs were found for the strongest avalanche peaks but not for the full sequence of avalanches. The memory effect is hence limited to strong avalanches

Magnetic Phase Transitions in NdCoAsO

Magnetization measurements reveal that NdCoAsO undergoes three magnetic phase transitions below room temperature. The crystal and magnetic structures of NdCoAsO have been determined by powder neutron diffraction, and the effects of the phase transitions on physical properties are reported. Near 69 K a ferromagnetic state emerges with a small saturation moment of about 0.2 Bohr magnetons, likely on Co atoms. At 14 K the material enters an antiferromagnetic state with propagation vector (0 0 1/2) and small ordered moments (~0.4 Bohr magnetons) on Co and Nd. Near 3.5 K a third transition is observed, and corresponds to the antiferromagnetic ordering, with the same propagation vector, of larger moments on Nd reaching 1.30(2) Bohr magnetons at 1.4 K. In addition, transport properties and heat capacity results are presented, and show anomalies at all three phase transitions.Comment: Some minor changes made, and lower temperature neutron diffraction results are included. Accepted for publication in Physical Review

Commensurate to incommensurate magnetic phase transition in Honeycomb-lattice pyrovanadate Mn2V2O7

We have synthesized single crystalline sample of Mn$_2$V$_2$O$_7$ using floating zone technique and investigated the ground state using magnetic susceptibility, heat capacity and neutron diffraction. Our magnetic susceptibility and heat capacity reveal two successive magnetic transitions at $T_{N1} =$ 19 K and $T_{N2} =$ 11.8 K indicating two distinct magnetically ordered phases. The single crystal neutron diffraction study shows that in the temperature ($T$) range 11.8 K $\le T \le$ 19 K the magnetic structure is commensurate with propagation vector $k_1 = (0, 0.5, 0)$, while upon lowering temperature below $T_{N2} =$ 11.8 K an incommensurate magnetic order emerges with $k_2 = (0.38, 0.48, 0.5)$ and the magnetic structure can be represented by cycloidal modulation of the Mn spin in $ac-$plane. We are reporting this commensurate to incommensurate transition for the first time. We discuss the role of the magnetic exchange interactions and spin-orbital coupling on the stability of the observed magnetic phase transitions.Comment: 8 pages, 7 figure

Complex and strongly anisotropic magnetism in the pure spin system EuRh2Si2

In divalent Eu systems, the 4f local moment has a pure spin state J=S=7/2. Although the absence of orbital moment precludes crystal electric field effects, we report a sizeable magnetic anisotropy in single crystals of EuRh2Si2. We observed a surprisingly complex magnetic behavior with three succesive phase transitions. The Eu2+ moments order in a likely amplitude-modulated structure below 24.5K, undergoing a further transition to a structure that is possibly of the equal moment type, and a first order transition at lower temperatures, presumably into a spin spiral structure. The sharp metamagnetic transition observed at low fields applied perpendicular to the hard axis is consistent with a change from a spiral to a fan structure. These magnetic structures are presumably formed by ferromagnetic planes perpendicular to the c axis, stacked antiferromagnetically along c but not of type I, at least just below the ordering temperature. Since EuRh2Si2 is isoelectronic to EuFe2As2, our results are also relevant for the complex Eu-magnetism observed there, especially for the transition from an antiferromagnetic to a ferromagnetic state observed in EuFe2P2 upon substituting As by P.Comment: submitted to Journal of Physics: Condensed Matte

First-order structural transition in the magnetically ordered phase of Fe1.13Te

Specific heat, resistivity, magnetic susceptibility, linear thermal expansion (LTE), and high-resolution synchrotron X-ray powder diffraction investigations of single crystals Fe1+yTe (0.06 < y < 0.15) reveal a splitting of a single, first-order transition for y 0.12. Most strikingly, all measurements on identical samples Fe1.13Te consistently indicate that, upon cooling, the magnetic transition at T_N precedes the first-order structural transition at a lower temperature T_s. The structural transition in turn coincides with a change in the character of the magnetic structure. The LTE measurements along the crystallographic c-axis displays a small distortion close to T_N due to a lattice striction as a consequence of magnetic ordering, and a much larger change at T_s. The lattice symmetry changes, however, only below T_s as indicated by powder X-ray diffraction. This behavior is in stark contrast to the sequence in which the phase transitions occur in Fe pnictides.Comment: 6 page