L. J. Chang

Higgs transition from a magnetic Coulomb liquid to a ferromagnet in Yb2Ti2O7

In a class of frustrated magnets known as spin ice, magnetic monopoles emerge as classical defects and interact via the magnetic Coulomb law. With quantum-mechanical interactions, these magnetic charges are carried by fractionalized bosonic quasi-particles, spinons, which can undergo Bose–Einstein condensation through a first-order transition via the Higgs mechanism. Here, we report evidence of a Higgs transition from a magnetic Coulomb liquid to a ferromagnet in single-crystal Yb2Ti2O7. Polarized neutron scattering experiments show that the diffuse [111]-rod scattering and pinch-point features, which develop on cooling are suddenly suppressed below TC~0.21 K, where magnetic Bragg peaks and a full depolarization of the neutron spins are observed with thermal hysteresis, indicating a first-order ferromagnetic transition. Our results are explained on the basis of a quantum spin-ice model, whose high-temperature phase is effectively described as a magnetic Coulomb liquid, whereas the ground state shows a nearly collinear ferromagnetism with gapped spin excitations.

Graphene quantum dots for valley-based quantum computing: A feasibility study

The rise of graphene opens a door to qubit implementation, as discussed in the recent proposal of valley pair qubits in double quantum dots of gapped graphene (G. Y. Wu, N.-Y. Lue, and L. Chang, arXiv:1104.0443). The work here presents the comprehensive theory underlying the proposal. It discusses the interaction of electrons with external magnetic and electric fields in such structures. Specifically, it examines a strong, unique mechanism, i.e., the analog of the first-order relativistic effect in gapped graphene. This mechanism is state mixing free and allows, together with electrically tunable exchange coupling, a fast, all-electric manipulation of qubits via electric gates, in the time scale of ns. The work also looks into the issue of fault tolerance in a typical case, yielding at 10 K a long qubit coherence time [\ensuremath{\sim}

Magnetic order in the double pyrochlore Tb2Ru2O7

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Static magnetic moments revealed by muon spin relaxation and thermodynamic measurements in the quantum spin ice Yb2 Ti2 O7

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Dielectric properties of the nanoporous MCM-41 matrix filled with the (NH4)2SO4 ferroelectric

Polycrystalline Tb(2)Ru(2)O(7) has been studied using dc susceptibility, specific heat and neutron scattering techniques. The high temperature paramagnetic state is dominated by the single ion character of Tb(3 + ) and very similar to that of the well-studied spin liquid Tb(2)Ti(2)O(7). However, both the Ru(4 + ) and Tb(3 + ) sublattices order, at about 110 K and 3.5 K, respectively. Although the Tb sublattice does not fully order until 3.5 K, it is polarized in the presence of the internal field generated by the Ru(4 + ) sublattice and possesses a significant moment at 7 K. Magnetic entropy measurements suggest that four levels exist in the first 30 K and inelastic neutron scattering investigations revealed two more levels at 10 and 14 meV. As the magnetic sublattices order, the excitations are perturbed from that measured in the paramagnetic state. These data are compared to data for other terbium based and double pyrochlores.

A compact SEOP 3He neutron spin filter with AFP NMR

We present muon spin relaxation (μSR) and specific-heat versus temperature C(T ) measurements on polycrystalline and single-crystal samples of the pyrochlore magnet Yb2Ti2O7. C(T ) exhibits a sharp peak at a TC of 0.21 and 0.26 K for the single-crystal and polycrystalline samples, respectively. For both samples, the magnetic entropy released between 50 mK and 30 K amounts to R ln 2 per Yb. At temperatures below TC we observe a steep drop in the asymmetry of the zero-field μSR time spectra at short time scales, as well as a decoupling of the muon spins from the internal field in longitudinal magnetic fields of 0.25 T for both the polycrystalline and single-crystal samples. These muon data are indicative of static magnetic moments. Our results are consistent with the onset of long-range magnetic order in both forms of Yb2Ti2O7.

Spin dynamics in Ho2Ru2O7

Variations with temperature of the linear dielectric permittivity and amplitude of the third harmonic were studied for nanoporous MCM-41 matrices with 4.0-nm channel pores filled with the (NH4)2SO4 ferroelectric, in comparison with bulk ammonium sulfate. The measurements were performed upon heating and cooling in the temperature range from 100 K to room temperature. A noticeable shift to low temperatures (by approximately 25 K) for the ferroelectric phase transition in the MCM-41/(NH4)2SO4 nanocomposite as compared to bulk (NH4)2SO4 was revealed. The temperature hysteresis observed at the phase transition in the nanocomposite was approximately 2 K which is close to that in bulk ammonium sulfate. The significant decrease of the transition temperature in nanostructured ammonium sulfate agrees with the theoretical predictions based on the Landau and Ising models of the size effect on the ferroelectric phase transition in isolated small particles.

Atomic structure study of the pyrochlore y b2 T i2 O7 and its relationship with low-temperature magnetic order

We developed AFP NMR in an aluminum container for polarized noble gas nuclei. The radio frequency magnetic field inside the aluminum container was designed from computer simulations. The polarization loss by the AFP spin flip of 3He was measured to be as low as 3.8×10−4. With this technique, a compact in-situ polarizing 3He neutron spin filter with AFP NMR is demonstrated.

Longitudinal and transverse Hall resistivities in NaFe1−xCoxAs single crystals with x = 0.022 and 0.0205: weak pinning and anomalous electrical transport properties

The spin relaxation processes within the pyrochlore Ho2Ru2O7 have been investigated by neutron scattering and bulk property techniques. A single-ion process, that is thermally activated, dominates the spin–spin relaxation spectrum above 2 K. Assuming Arrhenius behaviour, we found an activation energy Δ = (329 ± 6) K and characteristic relaxation time τ0 = (5.2 ± 0.3) × 10−12 s in the paramagnetic state, akin to those found in the spin ice, Ho2Ti2O7. Atlow temperature (T<95 K) the activation energy lowers and below 20 K the entropy and ac susceptibility are similar to that observed in other spin ice compounds within a 10 kOe field.

Low-energy excitations and ground-state selection in the quantum breathing pyrochlore antiferromagnet Ba3Yb2Zn5 O11

There has been great interest in the magnetic behavior of pyrochlore oxides with the general formula A2B2O7, in which rare-earth (A), and transition metal (B) cations are ordered on separate interpenetrating lattices of corner-sharing tetrahedra. Such materials exhibit behaviors including quantum spin-ice, (quantum) spin-liquid, and ordered magnetic ground states. Yb2Ti2O7 lies on the boundary between a number of competing magnetic ground states. Features in the low-temperature specific heat capacity that vary in sharpness and temperature from sample to sample suggest that in some cases the magnetic moments order, while in others the moments remain dynamic down to temperatures as low as ~16 mK. In this work, three different Yb2Ti2O7 samples, all grown by the optical floating zone technique but exhibiting quite different heat capacity behavior, are studied by aberration-corrected scanning transmission microscopy (STEM). Atomic-scale energy-dispersive X-ray analysis shows that a crystal with no specific heat anomaly has substitution of Yb atoms on Ti sites (‘stuffing’). We show that the detailed intensity distribution around the visible atomic columns in annular dark field STEM images is sensitive to the presence of nearby atoms of low atomic number (in this case oxygen) and find significant differences between the samples that correlates both with their magnetic behavior and measurements of Ti oxidation state using electron energy loss spectroscopy. These measurements support the view that the magnetic ground state of Yb2Ti2O7 is extremely sensitive to disorder.F. Delgado 2 and J. Fernández-Rossier 4 Centro de F́ısica de Materiales, CSIC-UPV/EHU, Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4-5, E-20018 Donostia-San Sebastián, Spain IKERBASQUE, Basque Foundation for Science, E-48013 Bilbao, Spain QuantaLab, International Iberian Nanotechnology Laboratory (INL), Av. Mestre José Veiga, 4715-310 Braga, Portugal Departamento de F́ısica Aplicada, Universidad de Alicante, Spain