^{59}Co NMR evidence for charge ordering below T_{CO}\sim 51 K in Na_{0.5}CoO_2

The CoO$_{2}$ layers in sodium-cobaltates Na$_{x}$CoO$_{2}$ may be viewed as a spin $S=1/2$ triangular-lattice doped with charge carriers. The underlying physics of the cobaltates is very similar to that of the high $T_{c}$ cuprates. We will present unequivocal $^{59}$Co NMR evidence that below $T_{CO}\sim51 K$, the insulating ground state of the itinerant antiferromagnet Na$_{0.5}$CoO$_{2}$ ($T_{N}\sim 86 K$) is induced by charge ordering.Comment: Phys. Rev. Lett. 100 (2008), in press. 4 figure

NMR Search for the Spin Nematic State in LaFeAsO Single Crystal

We report a 75-As single crystal NMR investigation of LaFeAsO, the parent phase of a pnictide high Tc superconductor. We demonstrate that spin dynamics develop a strong two-fold anisotropy within each orthorhombic domain below the tetragonal-orthorhombic structural phase transition at T[TO]~156 K. This intermediate state with a dynamical breaking of the rotational symmetry freezes progressively into a spin density wave (SDW) below T[SDW]~142 K. Our findings are consistent with the presence of a spin nematic state below T[TO] with an incipient magnetic order.Comment: Revised manuscript accepted for publication in Phys. Rev. Let

The Magnetic Properties of 1111-type Diluted Magnetic Semiconductor (La1−x_{1-x}Bax_{x})(Zn1−x_{1-x}Mnx_{x})AsO in the Low Doping Regime

We investigated the magnetic properties of (La$_{1-x}$Ba$_{x}$)(Zn$_{1-x}$Mn$_{x}$)AsO with $x$ varying from 0.005 to 0.05 at an external magnetic field of 1000 Oe. For doping levels of $x$ $\leq$ 0.01, the system remains paramagnetic down to the lowest measurable temperature of 2 K. Only when the doping level increases to $x$ = 0.02 does the ferromagnetic ordering appear. Our analysis indicates that antiferromagnetic exchange interactions dominate for $x$ $\leq$ 0.01, as shown by the negative Weiss temperature fitted from the magnetization data. The Weiss temperature becomes positive, i.e., ferromagnetic coupling starts to dominate, for $x$ $\geq$ 0.02. The Mn-Mn spin interaction parameter $\mid$$2J/k_B$$\mid$ is estimated to be in the order of 10 K for both $x$ $\leq$ 0.01 (antiferromagnetic ordered state) and $x$ $\geq$ 0.02 (ferromagnetic ordered state). Our results unequivocally demonstrate the competition between ferromagnetic and antiferromagnetic exchange interactions in carrier-mediated ferromagnetic systems.Comment: 9 pages, 3 figure

Biocompatible Mesoporous Hollow Carbon Nanocapsules for High Performance Supercapacitors.

A facile and general method for the controllable synthesis of N-doped hollow mesoporous carbon nanocapsules (NHCNCs) with four different geometries has been developed. The spheres (NHCNC-1), low-concaves (NHCNC-2), semi-concaves (NHCNC-3) and wrinkles (NHCNC-4) shaped samples were prepared and systematically investigated to understand the structural effects of hollow particles on their supercapacitor performances. Compared with the other three different shaped samples (NHCNC-1, NHCNC-2, and NHCNC-4), the as-synthesized semi-concave structured NHCNC-3 demonstrated excellent performance with high gravimetric capacitance of 326 F g-1 (419 F cm-3) and ultra-stable cycling stability (96.6% after 5000 cycles). The outstanding performances achieved are attributed to the unique semi-concave structure, high specific surface area (1400 m2 g-1), hierarchical porosity, high packing density (1.41 g cm-3) and high nitrogen (N) content (up to 3.73%) of the new materials. These carbon nanocapsules with tailorable structures and properties enable them as outstanding carriers and platforms for various emerging applications, such as nanoscale chemical reactors, catalysis, batteries, solar energy harvest, gas storage and so on. In addition, these novel carbons have negligible cytotoxicity and high biocompatibility for human cells, promising a wide range of bio applications, such as biomaterials, drug delivery, biomedicine, biotherapy and bioelectronic devices

Anisotropic Behavior of Knight Shift in Superconducting State of Na_xCoO_2yH_2O

The Co Knight shift was measured in an aligned powder sample of Na_xCoO_2yH_2O, which shows superconductivity at T_c \sim 4.6 K. The Knight-shift components parallel (K_c) and perpendicular to the c-axis (along the ab plane K_{ab}) were measured in both the normal and superconducting (SC) states. The temperature dependences of K_{ab} and K_c are scaled with the bulk susceptibility, which shows that the microscopic susceptibility deduced from the Knight shift is related to Co-3d spins. In the SC state, the Knight shift shows an anisotropic temperature dependence: K_{ab} decreases below 5 K, whereas K_c does not decrease within experimental accuracy. This result raises the possibility that spin-triplet superconductivity with the spin component of the pairs directed along the c-axis is realized in Na_xCoO_2yH_2O.Comment: 5 pages, 5 figures, to be published in Journal of Physical Society of Japan vol. 75, No.

(Sr3La2O5)(Zn1-xMnx)2As2: A Bulk Form Diluted Magnetic Semiconductor isostructural to the "32522" Fe-based Superconductors

A new diluted magnetic semiconductor system, (Sr3La2O5)(Zn1-xMnx)2As2, has been synthesized and characterized. 10% Mn substitution for Zn in bulk form (Sr3La2O5)Zn2As2 results in a ferromagnetic ordering below Curie temperature, TC ~ 40 K. (Sr3La2O5)(Zn1-xMnx)2As2 has a layered crystal structure identical to that of 32522-type Fe based superconductors, and represents the fifth DMS family that has a direct counterpart among the FeAs high temperature superconductor families.Comment: Accepted for publication in EP

Sodium vacancy ordering and the co-existence of localized spins and itinerant charges in NaxCoO2

The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides because the Co sits on a triangular lattice and its valence can be tuned over a wide range by varying the Na concentration x. Up to now detailed modeling of the rich phenomenology (which ranges from unconventional superconductivity to enhanced thermopower) has been hampered by the difficulty of controlling pure phases. We discovered that certain Na concentrations are specially stable and are associated with superlattice ordering of the Na clusters. This leads naturally to a picture of co-existence of localized spins and itinerant charge carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our picture brings coherence to a variety of measurements ranging from NMR to optical to thermal transport. Our results also allow us to take the first step towards modeling the mysterious ``Curie-Weiss'' metal state at x = 0.71. We suggest the local moments may form a quantum spin liquid state and we propose experimental test of our hypothesis.Comment: 16 pages, 5 figure