G. J. Shu

Searching for stable Na-ordered phases in single-crystal samples of γ-NaxCOO2

We report on the preparation and characterization of single-crystal {gamma} phase Na{sub x}CoO{sub 2} with 0.25{<=}x{<=}0.84 using a nonaqueous electrochemical chronoamperemetry technique. By carefully mapping the overpotential versus x (for x<0.84), we find six distinct stable phases with Na levels corresponding to x{approx} 0.75, 0.71, 0.50, 0.43, 0.33, and 0.25. The composition with x{approx_equal}0.55 appears to have a critical Na concentration which separates samples with different magnetic behavior as well as different Na ion diffusion mechanisms. Chemical analysis of an aged crystal reveals different Na ion diffusion mechanisms above and below x{sub c}{approx}0.53, where the diffusion process above x{sub c} has a diffusion coefficient about five times larger than that below x{sub c}. The series of crystals were studied with x-ray diffraction, susceptibility, and transport measurements. The crystal with x=0.5 shows a weak ferromagnetic transition below T=27 K in addition to the usual transitions at T=51 and 88 K. The resistivity of the Curie-Weiss metallic Na{sub 0.71}CoO{sub 2} composition has a very low residual resistivity, which attests to the high homogeneity of the crystals prepared by this improved electrochemical method. Our results on the various stable crystal compositions point to the importance of Na ion ordering across the phase diagram.

Sodium ion ordering and vacancy cluster formation in NaxCoO2 (x=0.71 and 0.84) single crystals by synchrotron X-Ray diffraction.

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 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.

Plasmons dispersion and nonvertical interband transitions in single crystal Bi2Se3 investigated by electron energy-loss spectroscopy

Plasmons dispersion and nonvertical interband transitions in Bi2Se3 single crystals were investigated by electron energy-loss spectroscopy in conjunction with (scanning) transmission electron microscopy [(S)TEM-EELS]. Both volume plasmons (π plasmon at 7 eV and π + σ plasmon at 17 eV) and surface plasmons (∼5.5 and 10 eV) were demonstrated in STEM-EELS spectra and the corresponding spectral imaging in real space. In further EELS experiments in reciprocal space, the momentum-dependent spectra reveal very different dispersion behavior between π and π + σ plasmons, with π + σ plasmons showing a typical quadratic dependence, whereas the π plasmon exhibited a linear dispersion analogous to what was reported for graphene. Furthermore, a low energy excitation 0.7–1.6 eV was also observed which is attributed to direct nonvertical interband transitions

Oxygen nonstoichiometry and the origin of Na ion ordering in P2-NaxCoO2

The impact of oxygen deficiency on physical properties of Na2/3CoO2-x has been investigated. From the combined thermogravimetric, magnetic susceptibility and synchrotron X-ray Laue diffraction studies, it is demonstrated that Na2/3CoO2 shows no superlattice ordering due to Na ions; however Na2/3CoO1.98, which has the same Co valence as that of Na0.71CoO2, shows nearly identical magnetic and transport properties and the same simple hexagonal superlattice ordering of sqrt(12)a. It is proposed that the Na ion ordering found in Na2/3CoO1.98 is identical to the ideal Na0.71CoO2 of large sqrt(12)a x sqrt(12)a x 3c superlattice but with additional Na vacancies which are bound to the oxygen defects at room temperature. We conclude that oxygen vacancies play a key role in stabilizing the superlattice structure and must be accounted for in its modeling.

Electronic phase diagram of Li xCoO 2 revisited with potentiostatically deintercalated single crystals

Electronic phase diagram of LixCoO2 has been re-examined using potentiostatically de-intercalated single crystal samples. Stable phases of x ~ 0.87, 0.72, 0.53, 0.50, 0.43, and 0.33 were found and isolated for physical property studies. A-type and chain-type antiferromagnetic orderings have been suggested from magnetic susceptibility measurement results in x ~ 0.87 and 0.50 below ~ 10 K and 200 K, respectively, similar to those found in NaxCoO2 system. There is no Li vacancy superlattice ordering observed at room temperature for the electronically stable phase Li0.72CoO2 as revealed by synchrotron X-ray Laue diffraction. The peculiar magnetic anomaly near ~ 175 K as often found in powder samples of x ~ 0.46-0.78 cannot be isolated through this single crystal potentiostatic method, which supports the previously proposed explanation to be surface stabilized phase of significant thermal hysteresis and aging character.

Local Moment, itinerancy, and deviation from Fermi-liquid behavior in NaxCoO2 for 0.71 <= x <= 0.84

Here we report the observation of Fermi surface (FS) pockets via the Shubnikov-de Haas effect in NaxCoO2 for x=0.71 and 0.84, respectively. Our observations indicate that the FS expected for each compound intersects their corresponding Brillouin zones, as defined by the previously reported superlattice structures, leading to small reconstructed FS pockets, but only if a precise number of holes per unit cell is localized. For 0.71< or = x < 0.75 the coexistence of itinerant carriers and localized S=1/2 spins on a paramagnetic triangular superlattice leads at low temperatures to the observation of a deviation from standard Fermi-liquid behavior in the electrical transport and heat capacity properties, suggesting the formation of some kind of quantum spin-liquid ground state.

Magnetic ordering and dielectric relaxation in the double perovskite YBaCuFeO5

Using magnetization, dielectric constant, and neutron diffraction measurements on a high quality single crystal of YBaCuFeO5 (YBCFO), we demonstrate that the crystal shows two antiferromagnetic transitions at [Formula: see text] K and [Formula: see text] K, and displays a giant dielectric constant with a characteristic of the dielectric relaxation at T N2. It does not show the evidence of the electric polarization for the crystal used for this study. The transition at T N1 corresponds with a paramagnetic to antiferromagnetic transition with a magnetic propagation vector doubling the unit cell along three crystallographic axes. Upon cooling, at T N2, the commensurate spin ordering transforms to a spiral magnetic structure with a propagation vector of ([Formula: see text] [Formula: see text] [Formula: see text]), where [Formula: see text], [Formula: see text], and [Formula: see text] are odd, and the incommensurability δ is temperature dependent. Around the transition boundary at T N2, both commensurate and incommensurate spin ordering coexist.

Dynamic susceptibility study on the skyrmion phase stability of Fe0.7Co0.3Si

The AC susceptibilities of Fe1−xCoxSi (0.1 ≤ x ≤ 0.7) alloys were measured and compared with that of MnSi. The range of skyrmion phase in the H-T phase space was enlarged and moved to higher temperatures when approximately 1/3 of the Fe was substituted with Co. Comparing with MnSi, the skyrmion phase of Fe0.7Co0.3Si was found thermodynamically more stable under a much lower and narrower critical field near 280 ± 50 Oe. The stability range in the H-T phase space and the free energy reduction ΔE (as a function of the applied magnetic field) for both the Fe0.7Co0.3Si and MnSi were compared.

59Co NMR evidence for charge ordering below T_(CO) approximately 51 K in Na0.5CoO2.

The CoO2 layers in NaxCoO2 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 59Co NMR evidence that below T_(CO) approximately 51 K, the insulating ground state of the itinerant antiferromagnet Na0.5CoO2 (T_(N) approximately 86 K) is induced by charge ordering.

Mn vacancy defects, grain boundaries, and A-phase stability of helimagnet MnSi

Mn vacancy defect and grain size are shown to modify the magnetic phase diagram of MnSi significantly, especially near the critical regime of A-phase (skyrmion lattice) formation and the helimagnetic phase transition. Crystals grown using controlled nonstoichiometric initial precursors creates both grain boundaries and intrinsic Mn vacancy defect of various levels in MnSi. The results of combined transport, specific heat, and AC spin susceptibility measurements are compared for MnSi single crystal samples of various manganese deficiency levels and grain sizes. The finite-size effect and Mn vacancy level dependent helical phase transition temperature T(c) have been identified and verified. The stability of A-phase in H-T phase space has been examined through AC spin susceptibility data analysis.