Chris Baines

Magnetic quantum critical point and superconductivity in UPt3 doped with Pd.

Transverse-field muon spin relaxation measurements have been carried out on the heavy-fermion superconductor UPt (3) doped with small amounts of Pd. We find that the critical Pd concentration for the emergence of the large-moment antiferromagnetic phase is approximately 0.6 at. %Pd. At the same Pd content, superconductivity is completely suppressed. The existence of a magnetic quantum critical point in the phase diagram, which coincides with the critical point for superconductivity, provides evidence for ferromagnetic spin-fluctuation mediated odd-parity superconductivity, which competes with antiferromagnetic order.

Short-range correlations in the magnetic ground state of na4ir3 O8

The magnetic ground state of the J(eff)=1/2 hyperkagome lattice in Na₄Ir₃O₈ is explored via combined bulk magnetization, muon spin relaxation, and neutron scattering measurements. A short-range, frozen state comprised of quasistatic moments develops below a characteristic temperature of T(F)=6u2009u2009K, revealing an inhomogeneous distribution of spins occupying the entirety of the sample volume. Quasistatic, short-range spin correlations persist until at least 20 mK and differ substantially from the nominally dynamic response of a quantum spin liquid. Our data demonstrate that an inhomogeneous magnetic ground state arises in Na₄Ir₃O₈ driven either by disorder inherent to the creation of the hyperkagome lattice itself or stabilized via quantum fluctuations.

Dynamics at T\0 in half-integer isotropic high-spin molecules

We investigate the dynamical spin-spin autocorrelation function of the isotropic high-spin molecules CrCu 6 (S=9/2), CrNi 6 (S=15/2), and CrMn 6 (S=27/2), using magnetization, spin-lattice relaxation of muon spin (μSR), and NMR measurements. We find that the field autocorrelation time Τ of the molecules spin at zero and low fields is nearly temperature independent as T→50 mK. The high temperature Τ is very different between the three molecules. Surprisingly, it is identical (∼10 ns) at low temperature. This suggests that τ is governed by hyperfine interactions.

Low-temperature magnetic fluctuations in the Kondo insulator SmB6

We present the results of a systematic investigation of the magnetic properties of the three-dimensional Kondo topological insulator SmB6 using magnetization and muon-spin relaxation/rotation (μSR) measurements. The μSR measurements exhibit magnetic field fluctuations in SmB6 below ∼15 K due to electronic moments present in the system. However, no evidence for magnetic ordering is found down to 19 mK. The observed magnetism in SmB6 is homogeneous in nature throughout the full volume of the sample. Bulk magnetization measurements on the same sample show consistent behavior. The agreement between μSR, magnetization, and NMR results strongly indicate the appearance of intrinsic bulk magnetic in-gap states associated with fluctuating magnetic fields in SmB6 at low temperature.

Microscopic study of the superconducting state of the iron pnictide RbFe_{2}As_{2} via muon spin rotation

A study of the temperature and field dependence of the penetration depth λ of the superconductor RbFe2As2(Tc=2.52u2002K) was carried out by means of muon-spin rotation measurements. In addition to the zero-temperature value of the penetration depth λ(0)=267(5)u2002nm, a determination of the upper critical field Bc2(0)=2.6(2)u2002T was obtained. The temperature dependence of the superconducting carrier concentration is discussed within the framework of a multigap scenario. Compared to the other “122” systems which exhibit much higher Fermi level, a strong reduction in the large gap BCS ratio 2Δ/kBTc is observed. This is interpreted as a consequence of the absence of interband processes. Indications of possible pair-breaking effect are also discussed. n n© 2010 The American Physical Society

Conventional superconductivity and charge-density-wave ordering in Ba1−xNaxTi2Sb2O

We have investigated the low-temperature physical properties of BaTi2Sb2O and Ba1−xNaxTi2Sb2O (x=0.05, 0.1, 0.15, 0.2, 0.25, 0.3) by means of muon spin rotation (μSR) and SQUID magnetometry. Our measurements reveal the absence of magnetic ordering below TDW=58 K in the parent compound. Therefore the phase transition at this temperature observed by magnetometry is most likely due to the formation of a charge-denisty wave (CDW). Upon substitution of barium by sodium in Ba1−xNaxTi2Sb2O we find for x=0.25 superconductivity with a maximum Tc=5.1 K in the magnetization and a bulk Tc,bulk=4.5 K in the μSR measurements. The temperature dependency of the London penetration depth λ−2(T) of the optimally doped compound can be well explained within a conventional weak-coupling scenario in the clean limit.

Magnetic order in quasi-two-dimensional molecular magnets investigated with muon-spin relaxation

We present the results of a muon-spin relaxation ({mu}+SR) investigation into magnetic ordering in several families of layered quasi-two-dimensional molecular antiferromagnets based on transition-metal ions such as S = 1/2 Cu{sup 2+} bridged with organic ligands such as pyrazine. In many of these materials magnetic ordering is difficult to detect with conventional magnetic probes. In contrast, {mu}{sup +}SR allows us to identify ordering temperatures and study the critical behavior close to TN. Combining this with measurements of in-plane magnetic exchange J and predictions from quantum Monte Carlo simulations we may assess the degree of isolation of the 2D layers through estimates of the effective inter-layer exchange coupling and in-layer correlation lengths at TN. We also identify the likely metal-ion moment sizes and muon stopping sites in these materials, based on probabilistic analysis of the magnetic structures and of muon-fluorine dipole-dipole coupling in fluorinated materials.

Gapless excitations in the ground state of 1T−TaS2

1T-TaS

Coulomb spin liquid in anion-disordered pyrochlore Tb2Hf2O7

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Control of the third dimension in copper-based square-lattice antiferromagnets

is a layered transition metal dichalgeonide with a very rich phase diagram. At T=180K it undergoes a metal to Mott insulator transition. Mott insulators usually display anti-ferromagnetic ordering in the insulating phase but 1T-TaS