arXiv subjects

F. C. Chou

Publications and source records attributed to F. C. Chou.

At least 109 records · Page 6Linked to original sources

arXiv2004

Magnetic-field-induced Fermi surface reconstruction in Na$_{0.5}$CoO$_2$

We have performed electrical transport measurements at low temperatures and high magnetic fields in Na$_{0.5}$CoO$_2$ single crystals. Shubnikov de Haas oscillations were observed for two frequencies F_1 150 and F_2 40 T corresponding respectively to 1 and .25% of the area of the orthorhombic Brillouin zone. These small Fermi surface (FS) pockets indicate that most of the original FS vanishes at the charge ordering (CO) transition. Furthermore, in-plane magnetic fields strongly suppress the CO state. For fields rotating within the conducting planes we observe angular magnetoresistance oscillations (AMRO), whose periodicity changes from two- to six-fold at the transition, suggesting that a reconstructed hexagonal FS emerges at a field of about 40 T.

cond-mat.str-el↗

arXiv2004

Spin-Dynamics in the Carrier-Doped $S=1/2$ Triangular-Lattice of Na$_{x}$CoO$_{2}$-yH$_{2}$O

We probed the local electronic properties of the mixed-valent Co(+4-x) triangular-lattice in Na{x}CoO{2}-yH{2}O by 59-Co NMR. We observed two distinct types of Co sites for x>=1/2, but the valence seems averaged out for x~1/3. Local spin fluctuations exhibit qualitatively the same trend down to ~100 K regardless of the carrier-concentration x, and hence the nature of the electronic ground state. A canonical Fermi-liquid behavior emerges below ~100 K only for x~1/3.

cond-mat.str-el↗

arXiv2004

Orbital Symmetry and Electron Correlation in Na_{x}CoO_2

Measurements of polarization-dependent soft x-ray absorption reveal that the electronic states determining the low-energy excitations of Na$_{x}$CoO$_2$ have predominantly $a_{1g}$ symmetry with significant O $2p$ character. A large transfer of spectral weight observed in O $1s$ x-ray absorption provides spectral evidence for strong electron correlations in the layered cobaltates. Comparing Co $2p$ x-ray absorption with calculations based on a cluster model, we conclude that Na$_{x}$CoO$_2$ exhibits a charge-transfer electronic character rather than a Mott-Hubbard character.

cond-mat.str-el↗

arXiv2004

Molecular orbital excitations in cuprates

We report resonant inelastic x-ray scattering studies of electronic excitations in a wide variety of cuprate compounds. Specifically, we focus on the charge-transfer type excitation of an electron from a bonding molecular orbital to an antibonding molecular orbital in a copper oxygen plaquette. Both the excitation energy and the amount of dispersion are found to increase significantly as the copper oxygen bond-length is reduced. We also find that the estimated bond-length dependence of the hopping integral t_pd is much stronger than that expected from tight-binding theory.

cond-mat.str-el↗

arXiv2004

Magnetic susceptibility study of hydrated and non-hydrated NaxCoO2-yH2O single crystals

We have measured the magnetic susceptibility of single crystal samples of non-hydrated NaxCoO2 (x ~ 0.75, 0.67, 0.5, and 0.3) and hydrated Na0.3CoO2-yH2O (y ~ 0, 0.6, 1.3). Our measurements reveal considerable anisotropy between the susceptibilities with H||c and H||ab. The derived anisotropic g-factor ratio (g_ab/g_c) decreases significantly as the composition is changed from the Curie-Weiss metal with x = 0.75 to the paramagnetic metal with x = 0.3. Fully hydrated Na0.3CoO2-1.3H2O samples have a larger susceptibility than non-hydrated Na0.3CoO2 samples, as well as a higher degree of anisotropy. In addition, the fully hydrated compound contains a small additional fraction of anisotropic localized spins.

cond-mat.str-el↗

arXiv2004

Structural transition in NaxCoO2 with x near 0.75 due to Na rearrangement

We report neutron powder diffraction measurements on a series of NaxCoO2 samples with x near 0.75 which were prepared under different synthesis conditions. Two different crystal structures for the samples are observed at room temperature. The two structures differ primarily by a shift of a large fraction of the Na ions from a high-symmetry position to a lower-symmetry position. Close inspection of the refinement parameters indicates that the presence of either structure depends sensitively on the Na content x, with x = 0.75 as the critical concentration separating the two phases. By raising the temperature to around T = 323 K, the high-symmetry structure can be converted to the lower-symmetry structure. The transition is reversible, but there is significant hysteresis. We discuss the effects of this structural transition on the bulk magnetic and transport properties.

cond-mat.str-el↗

arXiv2004

Electrochemical De-intercalation, Oxygen Non-stoichiometry, and Crystal Growth of NaxCoO2-d

We report a detailed study of de-intercalation of Na from the compound NaxCoO2-d using an electrochemical technique. We find evidence for stable phases with Na contents near the fractions ~1/3, 1/2, 5/8, 2/3, and 3/4. Details regarding the floating-zone crystal growth of Na0.75CoO2 single crystals are discussed as well as results from magnetic susceptibility measurements. We observe the presence of significant oxygen deficiencies in powder samples of Na0.75CoO2-d prepared in air, but not in single crystal samples prepared in an oxygen atmosphere. The oxygen deficiencies in a Na0.75CoO2-d sample with d ~ 0.08 remain even after electrochemically de-intercalating to Na0.3CoO2-d.

cond-mat.str-el↗

arXiv2004

Analysis of the phonon spectrum in the titanium oxyhalide TiOBr

We present the electrodynamic response of TiOBr, which undergoes a transition to a spin-singlet ground state for temperatures below T$_{C1}=28$ K. The temperature evolution of the phonon anomalies indicates, like in \cl, an extended fluctuation regime, extending well above T$_{C1}$. At low frequencies, the spectral weight is progressively suppressed by decreasing the temperature, suggesting the formation of a spin-gap and a considerable electron-phonon coupling. A comparison of the two oxyhalides shows a weaker interplane coupling and reduced dimensionality for TiOBr that enhances fluctuations and shifts the ordering temperatures to lower values. In the fluctuation regime the temperature dependencies of some phonon parameters track the temperature evolution of the magnetic susceptibility.

cond-mat.mtrl-sci↗

arXiv2003

Thermodynamic and Transport Measurements on Superconducting Na_xCoO_2 + yH_2O Single Crystals Prepared by Electrochemical De-intercalation

Superconducting single crystal samples of Na_xCoO_2 + yH_2O have been produced using an electrochemical technique which dispenses with the usual bromine chemical de-intercalation step and permits much more precise control of the Na content. After careful hydration, nearly single-phase crystals have been obtained in which over 90% of the sample's volume corresponds to the superconducting Na_{0.3}CoO_2 + 1.3H_2O structure. Susceptibility and specific heat measurements confirm that bulk superconductivity has been achieved. The extracted normal state density of states indicates Fermi-liquid behavior with strong mass enhancement and a modest Wilson ratio. Measurements of H_{c2} for H||c and H||ab reveal significant anisotropy. The estimated value of H_{c2} for H||c yields a coherence length of ~100 A, consistent with an extremely narrow bandwidth.

cond-mat.str-el↗

arXiv2003

Neutron scattering study of the effects of dopant disorder on the superconductivity and magnetic order in stage-4 La_2CuO_{4+y}

We report neutron scattering measurements of the structure and magnetism of stage-4 La_2CuO_{4+y} with T_c ~42 K. Our diffraction results on a single crystal sample demonstrate that the excess oxygen dopants form a three-dimensional ordered superlattice within the interstitial regions of the crystal. The oxygen superlattice becomes disordered above T ~ 330 K, and a fast rate of cooling can freeze-in the disordered-oxygen state. Hence, by controlling the cooling rate, the degree of dopant disorder in our La_2CuO_{4+y} crystal can be varied. We find that a higher degree of quenched disorder reduces T_c by ~ 5 K relative to the ordered-oxygen state. At the same time, the quenched disorder enhances the spin density wave order in a manner analogous to the effects of an applied magnetic field.

cond-mat.str-el↗

arXiv2003

Effect of Na content and hydration on the excitation spectrum of the cobaltite Na_xCoO_2 yH_2O

We report on a Raman scattering study on the superconducting cobaltite $Na_xCoO_2\cdot yH_2O$ as function of Na content and hydration (x$\approx$1/3, 3/4 and y$\approx$0, 2/3, 4/3). The observed phonon scattering and scattering continua are analyzed in terms of lattice strain due to the structural misfit and disorder. Hydration, due to the intercalation of one or two $H_2O$ layers, releases a part of this strain. Our Raman data suggest a connection between disorder on the partly occupied Na sites, the split off of the $a_{1g}$ level from the other $t_{2g}$ states of $Co^{4+}$ and superconductivity.

cond-mat.str-el↗

arXiv2003

Infrared optical properties of the spin-1/2 quantum magnet $TiOCl$

We report results on the electrodynamic response of $TiOCl$, a low-dimensional spin-1/2 quantum magnet that shows a spin gap formation for T$<T_{c1}$= 67 $K$. The Fano-like shape of a few selected infrared active phonons suggests an interaction between lattice vibrations and a continuum of low frequency (spin) excitations. The temperature dependence of the phonon mode parameters extends over a broad temperature range well above $T_{c1}$, indicating the presence of an extended fluctuation regime. In the temperature interval between 200 $K$ and $T_{c1}$ there is a progressive dimensionality crossover (from two to one), as well as a spectral weight shift from low towards high frequencies. This allows us to identify a characteristic energy scale of about 430 $K$, ascribed to a pseudo spin-gap.

cond-mat.str-el↗

arXiv2003

Giant phonon anomalies in the pseudo-gap phase of TiOCl

We report infrared and Raman spectroscopy results of the spin-1/2 quantum magnet TiOCl. Giant anomalies are found in the temperature dependence of the phonon spectrum, which hint to unusual coupling of the electronic degrees of freedom to the lattice. These anomalies develop over a broad temperature interval, suggesting the presence of an extended fluctuation regime. This defines a pseudo-gap phase, characterized by a local spin-gap. Below 100 K a dimensionality cross-over leads to a dimerized ground state with a global spin-gap of about 2$Δ_{spin}\approx$~430 K.

cond-mat.str-el↗

arXiv2003

Charge and orbital excitations in Li2CuO2

We report a resonant inelastic x-ray scattering study of electronic excitations in Li_2CuO_2, an insulating compound comprised of ribbons of edge-sharing copper-oxygen chains. Three excitations, which show little dependence on momentum transfer, are observed in our measurements. The lowest energy excitation at ~2.1 eV is dispersionless and is attributed to a localized d-d orbital excitation. We also observe two excitations at ~5.4 eV and ~7.6 eV which we assign to charge-transfer excitations. These high-energy excitations are also dispersionless along the copper-oxygen chain direction. However, in each case we observe a small energy dispersion along the direction perpendicular to the copper-oxygen ribbons, suggesting a significant interchain coupling in this system. We also discuss the possible implications of ferromagnetic nearest-neighbor intrachain coupling on the charge excitation spectra.

cond-mat.str-el↗

arXiv2003

Effect of a magnetic field on long-range magnetic order in stage-4 and stage-6 superconducting La2CuO(4+y)

We have measured the enhancement of the static incommensurate spin-density wave (SDW) order by an applied magnetic field in stage-4 and stage-6 samples of superconducting La2CuO(4+y). We show that the stage-6 La2CuO(4+y) (Tc=32 K) forms static long-range SDW order with the same wave-vector as that in the previously studied stage-4 material. We have measured the field dependence of the SDW magnetic Bragg peaks in both stage-4 and stage-6 materials at fields up to 14.5 T. A recent model of competing SDW order and superconductivity describes these data well.

cond-mat.str-el↗

arXiv2003

Novel Spin-Gap Behavior in Layered S=1/2 Quantum Spin System TiOCl

We have investigated the spin fluctuations and their interplay with lattice instabilities in a layered S=1/2 quantum spin system TiOCl. Our Ti and Cl NMR data demonstrate a pseudo spin-gap behavior below T*=135 K, followed by successive phase transitions at T_c1=94K and T_c2=66K into the singlet ground state with an unusually large energy gap E_g=430K. The broad distribution of the local electric field gradient in the intermediate phase between T_c1 and T_c2 suggests unconventional spin and lattice (probably orbital) states with an emerging spin-gap.

cond-mat.str-el↗

arXiv2002

S=1/2 chains and spin-Peierls transition in TiOCl

We study TiOCl as an example of an S=1/2 layered Mott insulator. From our analysis of new susceptibility data, combined with LDA and LDA+U band structure calculations, we conclude that orbital ordering produces quasi-one-dimensional spin chains and that TiOCl is a new example of Heisenberg-chains which undergo a spin-Peierls transition. The energy scale is an order of magnitude larger than that of previously known examples. The effects of non-magnetic Sc impurities are explained using a model of broken finite chains.

cond-mat.str-el↗

arXiv2002

17O NMR Study of the Local Charge State in the Hole Doped Cu2O3 Two-Leg Spin-Ladder A14Cu24O41 (A=La6Ca8, Sr14, Sr11Ca3, Sr6Ca8)

NMR measurements of the ^{17}O nuclear quadrupole interactions, ^{17}ν_{Q}, show that the charge environment of the S=1/2 Cu_{2}O_{3} two-leg spin-ladder layer of hole-doped A_{14}Cu_{24}O_{41} changes dramatically above a certain temperature, T*. This temperature, T*, decreases with additional doping and is correlated with the magnetic crossover from the spin gap regime to the paramagnetic regime. We demonstrate that these changes in ^{17}ν_{Q} are consistent with an increase in the effective hole concentration in the Cu_{2}O_{3} two-leg ladder above T*. The effective hole concentration increases primarily in the oxygen 2pσorbitals.

cond-mat.str-el↗