Metallic quantum ferromagnets

An overview of quantum phase transitions (QPTs) in metallic ferromagnets, discussing both experimental and theoretical aspects, is given. These QPTs can be classified with respect to the presence and strength of quenched disorder: Clean systems generically show a discontinuous, or first-order, QPT from a ferromagnetic to a paramagnetic state as a function of some control parameter, as predicted by theory. Disordered systems are much more complicated, depending on the disorder strength and the distance from the QPT. In many disordered materials the QPT is continuous, or second order, and Griffiths-phase effects coexist with QPT singularities near the transition. In other systems the transition from the ferromagnetic state at low temperatures is to a different type of long-range order, such as an antiferromagnetic or a spin-density-wave state. In still other materials a transition to a state with glasslike spin dynamics is suspected. The review provides a comprehensive discussion of the current understanding of these various transitions and of the relation between experiment and theory.This work has been supported by the National Science Foundation under grant numbers NSF DMR-09-29966, DMR-09-01907, DMR-1401410, and DMR-1401449, and by the Deutsche Forschungsgemeinschaft under grant number FOR-960. Part of this work has been supported by the National Science Foundation under Grant. No. PHYS-1066293 and the hospitality of the Aspen Center for Physics

Composition dependence of bulk superconductivity in YFe2Ge2

In the layered iron-based superconductor YFe2Ge2, a high Sommerfeld ratio of ~100 mJ/molK^2 and a T^(3/2) temperature dependence of the electrical resistivity at low temperature T indicate strong electronic correlations and point towards an unconventional pairing state. We have investigated the role of composition and annealing conditions in optimizing the growth of high-quality YFe2Ge2. Our findings confirm that bulk superconductivity is observed in samples with disorder scattering rates less than 2 k_B T_c/hbar. Fe deficiency on the Fe site is identified as the dominant source of disorder, which can be minimised by precipitating from a slightly iron-rich melt, following by annealing

Evidence of a structural quantum critical point in (CaxSr1-x)3Rh4Sn13 from a lattice dynamics study

Approaching a quantum critical point (QCP) has been an effective route to stabilize superconductivity. While the role of magnetic QCPs has been extensively discussed, similar exploration of a structural QCP is scarce due to the lack of suitable systems with a continuous structural transition that can be conveniently tuned to 0~K. Using inelastic X-ray scattering, we examine the phonon spectrum of the nonmagnetic quasi-skutterudite (Ca$_{x}$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$, which represents a precious system to explore the interplay between structural instabilities and superconductivity by tuning the Ca concentration $x$. We unambiguously detect the softening of phonon modes around the M point on cooling towards the structural transition. Intriguingly, at $x=0.85$, the soft mode energy squared at the M point extrapolates to zero at $(-5.7 \pm 7.7)$~K, providing the first compelling microscopic evidence of a structural QCP in (Ca$_{x}$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$. The enhanced phonon density-of-states at low energy provides the essential ingredient for realizing strong-coupling superconductivity near the structural QCP

Superconductivity in diamond

We report the discovery of superconductivity in boron-doped diamond synthesized at high pressure (8-9 GPa) and temperature (2,500-2,800 K). Electrical resistivity, magnetic susceptibility, specific heat, and field-dependent resistance measurements show that boron-doped diamond is a bulk, type-II superconductor below the superconducting transition temperature Tc=4 K; superconductivity survives in a magnetic field up to Hc2(0)=3.5 T. The discovery of superconductivity in diamond-structured carbon suggests that Si and Ge, which also form in the diamond structure, may similarly exhibit superconductivity under the appropriate conditions.Comment: 13 pages, 4 figure

Shubnikov-de Haas measurements on LuRh2Si2

We present Shubnikov-de Haas measurements on LuRh2Si2, the non-magnetic reference compound to the prototypical heavy-fermion system YbRh2Si2. We find an extensive set of orbits with clear angular dependences. Surprisingly, the agreement with non-correlated band structure calculations is limited. This may be related to an uncertainty in the calculations arising from a lack of knowledge about the exact Si atom position in the unit cell. The data on LuRh2Si2 provide an extensive basis for the interpretation of measurements on YbRh2Si2 indicative of discrepancies between the high-field Fermi surface of YbRh2Si2 and the «small» Fermi surface configuration. © Published under licence by IOP Publishing Ltd

Muon spin rotation and relaxation study on Nb1-yFe2+y

We present a detailed study of the magnetic properties of weakly ferromagnetic/quantum critical Nb1-yFe2+y using muon spin rotation and relaxation (μSR). By means of an angular dependent study of the muon spin rotation signal in applied magnetic fields on a single crystal in the paramagnetic state we establish the muon stopping site in the crystallographic lattice of NbFe2. With this knowledge we develop models to describe the muon spin rotation and relaxation signals in the weakly ferromagnetic, spin density wave, and quantum critical phases of Nb1-yFe2+y and fit the corresponding experimental data. In particular, we quantify the μSR response for quantum critical behavior in Nb1.0117Fe1.9883 and extract the influence of residual weak structural disorder. From our analysis, Nb1-yFe2+y emerges to be uniquely suited to study quantum criticality close to weak itinerant ferromagnetic order