arXiv subjects

N. P. Ong

Publications and source records attributed to N. P. Ong.

124 records · Page 7Linked to original sources

arXiv2003

Anomalous high-temperature Hall effect on the triangular lattice in Na_xCoO_2

The Hall coefficient R_H of Na_xCoO_2 (x = 0.68) behaves anomalously at high temperatures. From 200 to 500 K, R_H increases linearly with T to 8 times the expected Drude value, with no sign of saturation. Together with the thermopower Q, the behavior of R_H provides firm evidence for strong correlation. We discuss the effect of hopping on a triangular lattice and compare R_H with a recent prediction by Kumar and Shastry.

cond-mat.supr-con↗

arXiv2003

Chemical Instability of the Cobalt Oxyhydrate Superconductor under Ambient Conditions

The layered sodium cobalt oxyhydrate superconductor Na0.3CoO2*1.4H2O is shown through X-ray diffraction and thermogravimetric studies to be one of a series of hydrated phases of Na0.3CoO2. Further, it is shown that the material is exceptionally sensitive to both temperature and humidity near ambient conditions, easily dehydrating to a non-superconducting lower hydrate. The observation of this stable lower hydrate with c=13.8 angstroms implies that the superconductivity turns on in this system between CoO2 layer spacings of 6.9 and 9.9 angstroms at nominally constant chemical doping.

cond-mat.supr-con↗

arXiv2002

Insulating behavior of lambda-DNA on the micron scale

We have investigated the electrical conductivity of lambda-DNA using DNA covalently bonded to Au electrodes. Thiol-modified dTTP was incorporated into the `sticky' ends of bacteriophage lambda-DNA using DNA polymerase. Two-probe measurements on such molecules provide a hard lower bound for the resistivity ρ>10^6 (Ohm cm) at bias potentials up to 20 volts, in conflict with recent claims of moderate to high conductivity. By direct imaging, we show that the molecules are present after the measurements. We stress the importance of eliminating salt residues in these measurements.

physics.bio-ph↗

arXiv2002

Collapse of the vortex-lattice inductance and shear modulus at the melting transition in untwinned $\rm YBa_2Cu_3O_7$

The complex resistivity $\hatρ(ω)$ of the vortex lattice in an untwinned crystal of 93-K $\rm YBa_2Cu_3O_7$ has been measured at frequencies $ω/2π$ from 100 kHz to 20 MHz in a 2-Tesla field $\bf H\parallel c$, using a 4-probe RF transmission technique that enables continuous measurements versus $ω$ and temperature $T$. As $T$ is increased, the inductance ${\cal L}_s(ω) ={\rm Im} \hatρ(ω)/ ω$ increases steeply to a cusp at the melting temperature $T_m$, and then undergoes a steep collapse consistent with vanishing of the shear modulus $c_{66}$. We discuss in detail the separation of the vortex-lattice inductance from the `volume' inductance, and other skin-depth effects. To analyze the spectra, we consider a weakly disordered lattice with a low pin density. Close fits are obtained to $ρ_1(ω)$ over 2 decades in $ω$. Values of the pinning parameter $κ$ and shear modulus $c_{66}$ obtained show that $c_{66}$ collapses by over 4 decades at $T_m$, whereas $κ$ remains finite.

cond-mat.supr-con↗

arXiv2002

The high-field phase diagram of the cuprates derived from the Nernst effect

Measurements of the Nernst signal in the vortex-liquid state of the cuprates to high fields (33 T) reveal that vorticity extends to very high fields even close to the zero-field critical temperature $T_{c0}$. In overdoped $\rm La_{2-x}Sr_xCuO_4$ (LSCO) we show that the upper critical field $H_{c2}(T)$ curve does not end at $T_{c0}$, but at a much higher temperature. These results imply that $T_{c0}$ corresponds to a loss in phase rigidity rather than a vanishing of the pairing amplitude. An intermediate field $H^*(T)\ll H_{c2}(T)$ is shown to be the field scale for the flux-flow resistivity.

cond-mat.supr-con↗

arXiv2001

Particle-hole symmetry in the antiferromagnetic state of the cuprates

In the layered cuprate perovskites, the occurence of high-temperature superconductivity seems deeply related to the unusual nature of the hole excitations. The limiting case of a very small number of holes diffusing in the antiferromagnetic (AF) background may provide important insights into this problem. We have investigated the transport properties in a series of crystals of $\rm YBa_2Cu_3O_y$, and found that the temperature dependences of the Hall coefficient $R_H$ and thermopower $S$ change abruptly as soon as the AF phase boundary is crossed. In the AF state at low temperatures $T$, both $R_H$ and $S$ are unexpectedly suppressed to nearly zero over a broad interval of $T$. We argue that this suppression arises from near-exact symmetry in the particle-hole currents. From the trends in $R_H$ and $S$, we infer that the symmetry is increasingly robust as the hole density $x$ becomes very small ($x\simeq 0.01$). We discuss implications for electronic properties both within the AF state and outside.

cond-mat.str-el↗

arXiv2001

The onset of the vortex-like Nernst signal above Tc in La_{2-x}Sr_xCuO_4 and Bi_2Sr_{2-y}La_yCuO_6

The diffusion of vortices down a thermal gradient produces a Josephson signal which is detected as the vortex Nernst effect. In a recent report, Xu et al., Nature 406, 486 (2000), an enhanced Nernst signal identified with vortex-like excitations was observed in a series of La_{2-x}Sr_xCuO_4 (LSCO) crystals at temperatures 50-100 K above T_c. To pin down the onset temperature T_ν of the vortex-like signal in the lightly doped regime (0.03 < x < 0.07), we have re-analyzed in detail the carrier contribution to the Nernst signal. By supplementing new Nernst measurements with thermopower and Hall-angle data, we isolate the off-diagonal Peltier conductivity α_{xy} and show that its profile provides an objective determination of T_ν. With the new results, we revise the phase diagram for the fluctuation regime in LSCO to accomodate the lightly doped regime. In the cuprate Bi_2Sr_{2-y}La_yCuO_6, we find that the carrier contribution is virtually negligible for y in the range 0.4-0.6. The evidence for an extended temperature interval with vortex-like excitations is even stronger in this system. Finally, we discuss how T_ν relates to the pseudogap temperature T* and the implications of strong fluctuations between the pseudogap state and the d-wave superconducting state.

cond-mat.str-el↗

arXiv2001

Low Temperature Fabrication of MgB2

We report the fabrication of MgB2 with bulk superconducting properties by conventional solid state methods at temperatures as low as 550 C. Mg deficiencies of the type Mg1-xB2 were tested. Tc was found to decrease by about 1K at large x, though the amount of non-stoichiometry, if any, is likely to be very small. For specific processing conditions, indications of the 25-30 K transition often seen in thin films were seen in the bulk materials. The lower temperature transition may be associated with the grain boundaries. These results indicate that it should be possible to fabricate MgB2 with bulk properties in in-situ thin films at temperatures of 600 C or less.

cond-mat.supr-con↗

arXiv2001

Superconductivity in the non-oxide Perovskite MgCNi3

The oxide perovskites are a large family of materials with many important physical properties. Of particular interest has been the fact that this structure type provides an excellent structural framework for the existence of superconductivity. The high Tc copper oxides are the most famous examples of superconducting perovskites, but there are many others [1]. Intermetallic compounds have been the source of many superconducting materials in the past, but they have been eclipsed in recent years by the perovskite oxides. The recent discovery of superconductivity in MgB2 [2] suggests that intermetallic compounds with simple structure types are worth serious reconsideration as sources of new superconducting materials. Here we report the observation of superconductivity at 8 K in the perovskite structure intermetallic compound MgCNi3, linking what appear at first sight to be mutually exclusive classes of superconducting materials. The observation of superconductivity in MgCNi3 indicates that MgB2 will not be the only one of its kind within the chemical paradigm that it suggests for new superconducting materials.

cond-mat.supr-con↗

arXiv2000

Giant enhancement of the thermal Hall conductivity κ_{xy} in the superconductor YBa_2Cu_3O_7

In high-purity crystals of YBa_2Cu_3O_7, the quasiparticle (qp) lifetime τand the (weak-field) thermal Hall conductivity κ_{xy} undergo dramatic increases below 90K. We present a detailed picture of the behavior of κ_{xy} at low temperature, in particular its scaling properties, which are directly relevant to the issue of whether Landau quantization of the qp states occurs.

cond-mat.supr-con↗

arXiv2000

Determining the Wiedemann-Franz ratio from the thermal Hall conductivity: Application to Cu and YBa_2Cu_3O_6.95

The Wiedemann-Franz (WF) ratio compares the thermal and electrical conductivities in a metal. We describe a new way to determine its value, based on the thermal Hall conductivity. The technique is applied to copper and to untwinned YBaCuO. In the latter, we uncover a T-linear dependence and suppression of the Hall-channel WF ratio. We discuss the implications of this suppressions. The general suppression of the WF ratio in systems with predominant electron-electron scattering is discussed.

cond-mat.supr-con↗

arXiv1999

Anisotropic effect of field on the orthorhombic-to-tetragonal transition in the striped cuprate (La,Nd)_{2-x}Sr_xCuO_4

The Nd-doped cuprate La_{2-y-x}Nd_ySr_xCuO_4 displays a first-order phase transition at T_d (= 74 K for x=0.10, y = 0.60) to a low-temperature tetragonal (LTT) phase. A magnetic field H applied || the a-axis leads to an increase in T_d, whereas T_d is decreased when H || c. These effects show that magnetic ordering involving both Nd and Cu spins plays a key role in driving the LTO-LTT transition. Related anisotropic effects are observed in the uniform susceptibility and the in-plane magnetoresistance.

cond-mat.supr-con↗

arXiv1999

Effect of the pseudogap on the Hall conductivity in underdoped YBa_2Cu_3O_6+x

In underdoped YBa_2Cu_3O_x (YBCO) with x = 6.63, the opening of the pseudogap at T^* ~ 160 K has a strong effect on the Hall angle tan(theta). While the Hall response is significantly reduced, the diagonal current is relatively unaffected. The Hall conductivity suppression continues deep into the flux-flow state (from T_c to 40 K), as an anomalous suppression of the vortex Hall current.

cond-mat.supr-con↗

arXiv1999

Quasiparticle thermal Hall angle and magnetoconductance in YBa_2Cu_3O_x

We present a way to extract the quasiparticle (qp) thermal conductivity Kappa_e and mean-free-path in YBa_2Cu_3O_x, using the thermal Hall effect and the magnetoconductance of Kappa_e. The results are very consistent with heat capacity experiments. Moreover, we find a simple relation between the thermal Hall angle Theta_Q and the H-dependence of Kappa_e, as well as numerical equality between Theta_Q and the electrical Hall angle. The findings also reveal an anomalously anisotropic scattering process in the normal state.

cond-mat.supr-con↗

arXiv1999

Thermal Conductivity as a Probe of Quasi-Particles in the Cuprates

In underdoped YBa_2Cu_3O_x (x=6.63), the low-T thermal conductivity Kappa_xx varies steeply with field B at small B, and saturates to a nearly field-independent value at high fields. The simple expression [1+p(T)|B|]^(-1) provides an excellent fit to Kappa_xx(B) over a wide range of fields. From the fit, we extract the zero-field mean-free-path, and the low temperature behavior of the QP current. The procedure also allows the QP Hall angle Theta_QP to be obtained. We find that Theta_QP falls on the 1/T^2 curve extrapolated from the electrical Hall angle above Tc. Moreover, it shares the same T dependence as the field scale p(T) extracted from Kappa_xx. We discuss implications of these results.

cond-mat.supr-con↗

arXiv1999

The Josephson Plasma Resonance in Bi_2Sr_2CaCu_2O_8 in a Tilted Field

The dependence of the Josephson plasma frequency omega_p in Bi_2Sr_2CaCu_2O_8 on a tilted field H is reported. Measurements over a large range of B and tilt angle theta allow a detailed comparison with a recent calculation by Koshelev. With a slight modification of the model, close agreement is obtained. From the fits, we find values for the in-plane correlation length and the zero-field critical current density J_{c0} (4,600 A/cm^2 at 30 K). An analogy to Bragg diffraction is described, as well as a picture for the fractional-exponent behavior of omega_p versus H.

cond-mat↗