Multiple phase transitions in single-crystalline Na1−δ_{1-\delta}FeAs

Specific heat, resistivity, susceptibility and Hall coefficient measurements were performed on high-quality single crystalline Na$_{1-\delta}$FeAs. This compound is found to undergo three successive phase transitions at around 52, 41, and 23 K, which correspond to structural, magnetic and superconducting transitions, respectively. The Hall effect result indicates the development of energy gap at low temperature due to the occurrence of spin-density-wave instability. Our results provide direct experimental evidence of the magnetic ordering in the nearly stoichiometric NaFeAs.Comment: 4 pages, 4 figure

The Range of Validity for the Kelvin Force

In a recent Letter, Luo, Du and Huang reported a novel convective instability driven by a force rarely studied before -- that exerted by an external magnetic field on a strongly magnetizable liquid. The associated physics is surprisingly rich and promises many more interesting results for the future. Unfortunately, the analysis starts from a misconception and employs the Kelvin force outside its range of validity. Since few would recognize this as a mistake, and since its consequence in the given experiment is particularly direct and critical, this is a point well worth being clarified, and clearly understood.Comment: 1 pag

Mixed-Spin Pairing Condensates in Heavy Nuclei

We show that the Bogoliubov-de Gennes equations for nuclear ground-state wave functions support solutions in which the condensate has a mixture of spin-singlet and spin-triplet pairing. We find that such mixed-spin condensates do not occur when there are equal numbers of neutrons and protons, but only when there is an isospin imbalance. Using a phenomenological Hamiltonian, we predict that such nuclei may occur in the physical region within the proton dripline. We also solve the Bogoliubov-de Gennes equations with variable constraints on the spin-singlet and spin-triplet pairing amplitudes. For nuclei that exhibit this new pairing behavior, the resulting energy surface can be rather soft, suggesting that there may be low-lying excitations associated with the spin mixing.Comment: 4+ pages, 3 figures, 1 table; 1 reference added; v2 corresponds to the published versio

Angular-dependent Magnetoresistance Oscillations in Na0.48_{0.48}CoO2_{2} Single Crystal

We report measurements of the c-axis angular-dependent magnetoresistance (AMR) for a Na$_{0.48}$CoO$_{2}$ single crystal, with a magnetic field of 10 T rotating within Co-O planes. Below the metal-insulator transition temperature induced by the charge ordering, the oscillation of the AMR is dominated by a two-fold rotational symmetry. The amplitudes of the oscillation corresponding to the four- and six-fold rotational symmetries are distinctive in low temperatures, but they merge into the background simultaneously at about 25 K. The six-fold oscillation originates naturally from the lattice symmetry. The observation of the four-fold rotational symmetry is consistent with the picture proposed by Choy, et al., that the Co lattice in the charge ordered state will split into two orthorhombic sublattice with one occupied by Co$^{3+}$ ions and the other by Co$^{4+}$ ions. We have also measured the c-axis AMR for Na$_{0.35}$CoO$_{2}$ and Na$_{0.85}$CoO$_{2}$ single crystals, and found no evidence for the existence of two- and four-fold symmetries.Comment: 4 pages, 6 figures. Submitted to PR

Beating of the oscillations in the transport coefficients of a one-dimensionally periodically modulated two-dimensional electron gas in the presence of spin-orbit interaction

Transport properties of a two-dimensional electron gas (2DEG) are studied in the presence of a perpendicular magnetic field $B$, of a {\it weak} one-dimensional (1D) periodic potential modulation, and of the spin-orbit interaction (SOI) described only by the Rashba term. In the absence of the modulation the SOI mixes the spin-up and spin-down states of neighboring Landau levels into two new, unequally spaced energy branches. The levels of these branches broaden into bands in the presence of the modulation and their bandwidths oscillate with the field $B$. Evaluated at the Fermi energy, the $n$-th level bandwidth of each series has a minimum or vanishes at different values of the field $B$. In contrast with the 1D-modulated 2DEG without SOI, for which only one flat-band condition applies, here there are two flat-band conditions that can change considerably as a function of the SOI strength $\alpha$ and accordingly influence the transport coefficients of the 2DEG. The phase and amplitude of the Weiss and Shubnikov-de Haas (SdH) oscillations depend on the strength $\alpha$. For small values of $\alpha$ both oscillations show beating patterns. Those of the former are due to the independently oscillating bandwidths whereas those of the latter are due to modifications of the density of states, exhibit an even-odd filling factor transition, and are nearly independent of the modulation strength. For strong values of $\alpha$ the SdH oscillations are split in two

Doping evoluton of antiferromagnetic order and structural distortion in LaFeAsO1−x_{1-x}Fx_x

We use neutron scattering to study the structural distortion and antiferromagnetic (AFM) order in LaFeAsO$_{1-x}$F$_{x}$ as the system is doped with fluorine (F) to induce superconductivity. In the undoped state, LaFeAsO exhibits a structural distortion, changing the symmetry from tetragonal (space group $P4/nmm$) to orthorhombic (space group $Cmma$) at 155 K, and then followed by an AFM order at 137 K. Doping the system with F gradually decreases the structural distortion temperature, but suppresses the long range AFM order before the emergence of superconductivity. Therefore, while superconductivity in these Fe oxypnictides can survive in either the tetragonal or the orthorhombic crystal structure, it competes directly with static AFM order.Comment: reference update

Fermi surface topology and low-lying quasiparticle structure of magnetically ordered Fe1+xTe

We report the first photoemission study of Fe1+xTe - the host compound of the newly discovered iron-chalcogenide superconductors. Our results reveal a pair of nearly electron- hole compensated Fermi pockets, strong Fermi velocity renormalization and an absence of a spin-density-wave gap. A shadow hole pocket is observed at the "X"-point of the Brillouin zone which is consistent with a long-range ordered magneto-structural groundstate. No signature of Fermi surface nesting instability associated with Q= pi(1/2, 1/2) is observed. Our results collectively reveal that the Fe1+xTe series is dramatically different from the undoped phases of the high Tc pnictides and likely harbor unusual mechanism for superconductivity and quantum magnetic order.Comment: 5 pages, 4 Figures; Submitted to Phys. Rev. Lett. (2009

Metric adjusted skew information: Convexity and restricted forms of superadditivity

We give a truly elementary proof of the convexity of metric adjusted skew information following an idea of Effros. We extend earlier results of weak forms of superadditivity to general metric adjusted skew informations. Recently, Luo and Zhang introduced the notion of semi-quantum states on a bipartite system and proved superadditivity of the Wigner-Yanase-Dyson skew informations for such states. We extend this result to general metric adjusted skew informations. We finally show that a recently introduced extension to parameter values $1<p\le 2$ of the WYD-information is a special case of (unbounded) metric adjusted skew information.Comment: An error in the literature is pointed ou