C. Broholm

Direct observation of field-induced incommensurate fluctuations in a one-dimensional S=1/2 antiferromagnet

Neutron scattering from copper benzoate, Cu(C6D5COO)(2) . 3D(2)O, provides the first direct experimental evidence for field-dependent incommensurate low energy modes in a one-dimensional spin S = 1/2 antiferromagnet. Soft modes occur for wave vectors (q) over tilde = pi +/- delta (q) over tilde(H), where delta (q) over tilde(H) approximate to 2 pi M(H)/g mu(B) as predicted by Bethe ansatz and spinon descriptions of the S = 1/2 chain. Unexpected was a field-induced energy gap Delta(H) alpha H-alpha, where alpha = 0.65(3) as determined from specific heat measurements. At H = 7 T (g mu(B)H/J = 0.52), the magnitude of the gap varies from 0.06J to 0.3J depending on the orientation of the applied field.

Control of Tetrahedral Coordination and Superconductivity in FeSe0.5Te0.5 Thin Films

We demonstrate a close relationship between superconductivity and the dimensions of the Fe-Se(Te) tetrahedron in FeSe0.5Te0.5. This is done by exploiting thin film epitaxy, which provides controlled biaxial stress, both compressive and tensile, to distort the tetrahedron. The Se/Te height within the tetrahedron is found to be of crucial importance to superconductivity, in agreement with the scenario that (π, π) spin fluctuations promote superconductivity in Fe superconductors.

Quantum fluctuations in spin-ice-like Pr2Zr2O7

Spin ice is a magnetic analog of H2O ice that harbors dense static disorder. Dipolar interactions between classical spins yield a frozen frustrated state with residual configurational Pauling entropy and emergent magnetic monopolar quasiparticles. Introducing quantum fluctuations is of great interest as this could melt spin ice and allow coherent propagation of monopoles. Here, we report experimental evidence for quantum dynamics of magnetic monopolar quasiparticles in a new class of spin ice based on exchange interactions, Pr2Zr2O7. Narrow pinch point features in otherwise diffuse elastic neutron scattering reflects adherence to a divergence-free constraint for disordered spins on long time scales. Magnetic susceptibility and specific heat data correspondingly show exponentially activated behaviors. In sharp contrast to conventional ice, however, >90% of the neutron scattering is inelastic and devoid of pinch points furnishing evidence for magnetic monopolar quantum fluctuations.

Extended quantum critical phase in a magnetized spin-1/2 antiferromagnetic chain.

Measurements are reported of the magnetic field dependence of excitations in the quantum critical state of the spin S=1/2 linear chain Heisenberg antiferromagnet copper pyrazine dinitrate (CuPzN). The complete spectrum was measured at k(B)T/J< or =0.025 for H=0 and H=8.7 T, where the system is approximately 30% magnetized. At H=0, the results are in agreement with exact calculations of the dynamic spin correlation function for a two-spinon continuum. At H=8.7 T, there are multiple overlapping continua with incommensurate soft modes. The boundaries of these continua confirm long-standing predictions, and the intensities are consistent with exact diagonalization and Bethe ansatz calculations.

Magnetic field splitting of the spin resonance in CeCoIn5.

Neutron scattering in strong magnetic fields is used to show the spin resonance in superconducting CeCoIn(5) (T(c)=2.3 K) is a doublet. The underdamped resonance (ħΓ=0.069±0.019 meV) Zeeman splits into two modes at E(±)=ħΩ(0)±αμ(B)μ(0)H with α=0.96±0.05. A linear extrapolation of the lower peak reaches zero energy at 11.2±0.5 T, near the critical field for the incommensurate Q phase. Kenzelmann et al. [Science 321, 1652 (2008)] This, taken with the integrated weight and polarization of the low-energy mode (E(-)), indicates that the Q phase can be interpreted as a Bose condensate of spin excitons.

Absence of low temperature anomaly in the Debye-Waller factor of solid He-4

The mean-square atomic displacement in hcp-phase solid {sup 4}He has been measured in crystals with a molar volume of 21.3 cm{sup 3}. It is temperature independent from 1 K to 140 mK, with no evidence for an anomaly in the vicinity of the proposed supersolid transition. The mean-square displacement is different for in-plane motions (0.122{+-}0.001 A{sup 2}) and out-of-plane motions (0.150{+-}0.001 A{sup 2})

Multiferroicity in the generic easy-plane triangular lattice antiferromagnet RbFe(MoO4)(2)

RbFe(MoO4)(2) is a quasi-two-dimensional (quasi-2D) triangular lattice antiferromagnet (TLA) that displays a zero-field magnetically driven multiferroic phase with a chiral spin structure. By inelastic neutron scattering, we determine quantitatively the spin Hamiltonian. We show that the easy-plane anisotropy is nearly 1/3 of the dominant spin exchange, making RbFe(MoO4)(2) an excellent system for studying the physics of the model 2D easy-plane TLA. Our measurements demonstrate magnetic-field-induced fluctuations in this material to stabilize the generic finite-field phases of the 2D XY TLA. We further explain how Dzyaloshinskii-Moriya interactions can generate ferroelectricity only in the zero-field phase. Our conclusion is that multiferroicity in RbFe(MoO4)(2), and its absence at high fields, results from the generic properties of the 2D XY TLA.

Neutron-Scattering Measurement of Incommensurate Short-Range Order in Single Crystals of the S = 1 Triangular Antiferromagnet NiGa2S4

C. Stock, S. Jonas, C. Broholm, 3 S. Nakatsuji, Y. Nambu, K. Onuma, Y. Maeno, and J.-H. Chung ISIS Facility, Rutherford Appleton Labs, Chilton, Didcot, OX11 0QX, UK Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland USA 21218 NIST Center for Neutron Research, Gaithersburg, Maryland USA 20899 Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan Department of Physics, Kyoto University, Kyoto 606-8502, Japan (Dated: June 17, 2010)

Unstable spin-ice order in the stuffed metallic pyrochlore Pr2+xIr2-xO7-δ

Specific heat, elastic neutron scattering, and muon spin rotation experiments have been carried out on a well-characterized sample of “stuffed” (Pr-rich) Pr2+xIr2-xO7-δ. Elastic neutron scattering shows the onset of long-range spin-ice “2-in/2-out” magnetic order at 0.93 kelvin, with an ordered moment of 1.7(1) Bohr magnetons per Pr ion at low temperatures. Approximate lower bounds on the correlation length and correlation time in the ordered state are 170 angstroms and 0.7 nanosecond, respectively. Muon spin rotation experiments yield an upper bound 2.6(7) milliteslas on the local field B4floc at the muon site, which is nearly two orders of magnitude smaller than the expected dipolar field for long-range spin-ice ordering of 1.7-Bohr magneton moments (120–270 milliteslas, depending on the muon site). This shortfall is due in part to splitting of the non-Kramers crystal-field ground-state doublets of near-neighbor Pr3+ ions by the positive-muon-induced lattice distortion. For this to be the only effect, however, ~160 Pr moments out to a distance of ~14 angstroms must be suppressed. An alternative scenario—one consistent with the observed reduced nuclear hyperfine Schottky anomaly in the specific heat—invokes slow correlated Pr-moment fluctuations in the ordered state that average B4floc on the μSR time scale (~10-7 second), but are staticmorexa0» on the time scale of the elastic neutron scattering experiments (~10-9 second). In this picture, the dynamic muon relaxation suggests a Pr3+ 4f correlation time of a few nanoseconds, which should be observable in a neutron spin echo experiment.«xa0less

Magnons and continua in a magnetized and dimerized spin - 12 chain

We examine the magnetic field dependent excitations of the dimerized spin -1/2 chain, copper nitrate, with antiferromagnetic intra-dimer exchange