Q. J. Li

A large iron isotope effect in SmFeAsO1-xFx and Ba1-xKxFe2As2

The recent discovery of superconductivity in oxypnictides with a critical transition temperature (TC) higher than the McMillan limit of 39 K (the theoretical maximum predicted by Bardeen–Cooper–Schrieffer theory) has generated great excitement. Theoretical calculations indicate that the electron–phonon interaction is not strong enough to give rise to such high transition temperatures, but strong ferromagnetic/antiferromagnetic fluctuations have been proposed to be responsible. Superconductivity and magnetism in pnictide superconductors, however, show a strong sensitivity to the crystal lattice, suggesting the possibility of unconventional electron–phonon coupling. Here we report the effect of oxygen and iron isotope substitution on TC and the spin-density wave (SDW) transition temperature (TSDW) in the SmFeAsO1 - xFx and Ba1 - xKxFe2As2 systems. The oxygen isotope effect on TC and TSDW is very small, while the iron isotope exponent αC = -dlnTC/dlnM is about 0.35 (0.5 corresponds to the full isotope effect). Surprisingly, the iron isotope exchange shows the same effect on TSDW as TC. This indicates that electron–phonon interaction plays some role in the superconducting mechanism, but a simple electron–phonon coupling mechanism seems unlikely because a strong magnon–phonon coupling is included.

Structure and physical properties for a new layered pnictide-oxide: BaTi2As2O

We have successfully synthesized a new layered pnictide-oxide: BaTi(2)As(2)O. It shares similar characteristics with Na(2)Ti(2)Sb(2)O. The crystal has a layered structure with a tetragonal P4/nmm group (a = 4.047(3) Å, c = 7.275(4) Å). The resistivity shows an anomaly at 200 K, which should be ascribed to an SDW or structural transition. The SDW or structural transition is confirmed by magnetic susceptibility and heat capacity measurements. These behaviors are very similar to those observed in parent compounds of high-T(c) iron-based pnictide superconductors, in which the superconductivity shows up when the anomaly due to the SDW or structural transition is suppressed. Therefore, the new layered pnictide-oxide, BaTi(2)As(2)O, could be a potential parent compound for superconductivity. It is found that Li( + ) doping significantly suppresses the anomaly, but no superconductivity emerges so far.

Ground state and magnetic phase transitions of orthoferriteDyFeO3

Low-temperature thermal conductivity (\kappa), as well as magnetization (M) and electric polarization (P), of multiferroic orthoferrite DyFeO_3 single crystals are studied with H \parallel c. When the crystal is cooled in zero field, M, P, and \kappa all consistently exhibit irreversible magnetic-field dependencies. In particular, with 500 mK < T \le 2 K, all these properties show two transitions at the first run of increasing field but only the higher-field transition is present in the subsequent field sweepings. Moreover, the ultra-low-T (T < 500 mK) \kappa(H) shows a different irreversibility and there is only one transition when the field is swept both up and down. All the results indicate a complex low-T H-T phase diagram involving successive magnetic phase transitions of the Fe^{3+} spins. In particular, the ground state, obtained with cooling to subKelvin temperatures in zero field, is found to be an unexplored phase.

Structure and physical properties of the new layered oxypnictides Sr4Sc2O6M2As2 (M=Fe and Co)

We have successfully prepared the new layered oxypnictides Sr4Sc2O6M2As2 (M=Fe and Co). They adopt the tetragonal structure, being the same as that of Sr4Sc2O6Fe2P2. The lattice constants are a=0.4045 nm and c=1.5802 nm for M=Fe, and a=0.4045 nm and c=1.5695 nm for M=Co, respectively. Their transport and magnetic properties have been systematically studied. The temperature dependence of the Hall coefficient and the thermoelectric power for the Sr4Sc2O6Fe2As2 compound show a complicated behavior, similar to that of the iron-based parent compounds LnOFeAs and BaFe2As2. This suggests that Sr4Sc2O6Fe2As2 could be considered as a new parent compound of iron-based superconductors.

Paramagnetic ground state with field-induced partial order in Nd3Ga5SiO14 probed by low-temperature heat transport

We study the low-temperature heat transport of Nd_3Ga_5SiO_{14}, which is a spin-liquid candidate, to probe the nature of ground state and the effect of magnetic field on the magnetic properties. The thermal conductivity (\kappa) shows a purely phononic transport in zero field. The external magnetic field along the c axis induces a dip-like behavior of \kappa(H), which can be attributed to a simple paramagnetic scattering on phonons. However, the magnetic field along the ab plane induces another step-like decrease of \kappa. This kind of \kappa(H) behavior is discussed to be related to a field-induced partial order, which yields low-energy magnetic excitations that significantly scatter phonons. These results point to a paramagnetic ground state that partial magnetic order can be induced by magnetic field along the ab plane, which is also signified by the low-T specific heat data.

Phonon-glass-like behavior of magnetic origin in single-crystal Tb 2 Ti 2 O 7

Q. J. Li, Z. Y. Zhao, C. Fan, F. B. Zhang, H. D. Zhou, 3 X. Zhao, ∗ and X. F. Sun † Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996-1200, USA National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32306-4005, USA School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China (Dated: May 30, 2013)

Low-temperature heat transport of Nd2CuO4: Roles of Nd magnons and spin-structure transitions

We report the magnetic-field dependence of thermal conductivity (\kappa) of an insulating cuprate Nd_2CuO_4 at very low temperatures down to 0.3 K. It is found that apart from the paramagnetic moments scattering on phonons, the Nd^{3+} magnons can act as either heat carriers or phonon scatterers, which strongly depends on the long-range antiferromagnetic transition and the field-induced transitions of spin structure. In particular, the Nd^{3+} magnons can effectively transport heat in the spin-flopped state of the Nd^{3+} sublattice. However, both the magnon transport and the magnetic scattering are quenched at very high fields. The spin re-orientations under the in-plane field can be conjectured from the detailed field dependence of \kappa.

Low-temperature heat transport in the layered spin-dimer compound Ba 3 Mn 2 O 8

We report a study on the low-temperature heat transport of the Ba

Low-temperature heat transport in the geometrically frustrated antiferromagnets R2Ti2O7 (R = Gd and Er)

{}_{3}

Superconductivity at 56 K in samarium-doped SrFeAsF

Mn