Searcharxiv⌕ Search

arXiv subjects

N. P. Ong

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

At least 109 records · Page 6Linked to original sources
arXiv2005

The Nernst effect in high-$T_c$ superconductors

The observation of a large Nernst signal $e_N$ in an extended region above the critical temperature $T_c$ in hole-doped cuprates provides evidence that vortex excitations survive above $T_c$. The results support the scenario that superfluidity vanishes because long-range phase coherence is destroyed by thermally-created vortices (in zero field), and that the pair condensate extends high into the pseudogap state in the underdoped (UD) regime. We present a series of measurements to high fields $H$ which provide strong evidence for this phase-disordering scenario.

cond-mat.supr-con↗
arXiv2005

Speeding up a single-molecule DNA motor with a simple catalyst

We demonstrate a catalytic control method for speeding up the single-molecule DNA motor introduced by Li and Tan [Nano Lett. {\bf 2}, 315 (2002)]. A key rate-limiting barrier in the reaction part of the cycle is the tendency for the second fuel strand $B$ to fold into the chair-like configuration of the original motor strand $M$. This seriously impedes the restoration reaction. We have designed a catalytic strand to inhibit the folding of $B$. Introduction of the catalyst speeds up the restoration reaction by roughly a factor of 2. The catalyst shows robust behavior for more than one cycle. The experimental data can be understood with a model with intermediate products. This technique provides dynamic control of the restoration rate of the motor without affecting the straightening rate.

cond-mat.soft↗
arXiv2005

Geometry and the anomalous Hall effet in ferromagnets

The geometric ideas underlying the Berry phase and the modern viewpoint of Karplus and Luttinger's theory of the anomalous Hall effect are discussed in an elementary way. We briefly review recent Hall and Nernst experiments which support the dominant role of the KL velocity term in ferromagnets.

cond-mat.other↗
arXiv2005

Giant Angular Dependent Nernst Effect in the Q1D Organic conductor (TMTSF)$_2$PF$_6$

We present a detailed study of the Nernst effect N$_{zx}$ in (TMTSF)$_2$PF$_6$ as a function of temperature, magnetic field magnitude and direction and pressure. As previously reported there is a large resonant-like structure as the magnetic field is rotated through crystallographic directions, the Lebed Magic Angles. These Nernst effect resonances strongly suggest that the transport of the system is effectively ``coherent'' only in crystallographic planes along or close to the applied field direction. We also present analytical and numerical calculations of the conductivity and thermoelectric tensors for (TMTSF)$_2$PF$_6$, based on a Boltzmann transport model within the semi-classical approximation. The Boltzmann transport calculation fails to describe the experiment data. We suggest that the answer may lie in field induced decoupling of the strongly correlated chains.

cond-mat.str-el↗
arXiv2005

Strongly nonlinear magnetization above $T_c$ in $\rm Bi_2Sr_2CaCu_2O_{8+δ}$

Using high-resolution magnetometry we have investigated in detail the magnetization $M$ above the critical temperature $T_c$ in $\rm Bi_2Sr_2CaCu_2O_{8+δ}$. In a broad range of temperature $T$ above $T_c$, we find that $M(T,H)$ is strongly non-linear in the field $H$. We show that as $T\to T_c$, the susceptibility $χ(T,H)$ diverges to very large values ($χ\to$ -1) if measured in weak $H$. In addition, $M(H)$ displays an anomalous non-analytic form $M\sim H^{1/δ}$ in weak fields with a strongly $T$-dependent exponent $δ(T)$. These features strongly support the proposal that, above $T_c$, the pair condensate survives to support significant London rigidity.

cond-mat.supr-con↗
arXiv2005

Structural and transport properties of epitaxial NaxCoO2 thin films

We have studied structural and transport properties of epitaxial NaxCoO2 thin films on (0001) sapphire substrate prepared by topotaxially converting an epitaxial Co3O4 film to NaxCoO2 with annealing in Na vapor. The films are c axis oriented and in-plane aligned with [10 1 0] NaxCoO2 rotated by 30 degrees from [10 1 0] sapphire. Different Na vapor pressures during the annealing resulted in films with different Na concentrations, which showed distinct transport properties.

cond-mat.str-el↗
arXiv2004

Anomalous-Hall heat current and Nernst effect in the ferromagnet CuCr_2Se_{4-x}Br_x

In a ferromagnet, an anomalous-Hall heat current, given by the off-diagonal Peltier term $α_{xy}$, accompanies the anomalous Hall current. By combining Nernst, thermopower and Hall experiments, we have measured how $α_{xy}$ varies with hole density and lifetime $τ$ in $\rm CuCr_2Se_{4-x}Br_x$. At low temperatures $T$, we find that $α_{xy}$ is independent of $τ$, consistent with anomalous-velocity theories. Its magnitude is fixed by a microscopic geometric area ${\cal A}\sim 34 \mathrmÅ^2$. Our results are incompatible with some models of the Nernst effect in ferromagnets.

cond-mat.other↗
arXiv2004

Superconductivity in three-layer Na0.3CoO2*1.3H2O

The observation of superconductivity at 4.3 K in a new crystalline form of Na0.3CoO2*1.3H2O is reported. The new superconductor has three layers of CoO6 octahedra per crystallographic unit cell, in contrast to the previously reported two-layer superconductor. The three-layer cell occurs because the relative orientations of neighboring CoO2 layers are distinctly different from what is seen in the two-layer superconducting phase. This type of structural difference in materials that are otherwise chemically and structurally identical is not possible to attain on the layered copper oxide superconductors. The synthesis and stability of the new phase are described.

cond-mat.supr-con↗
arXiv2004

Spin entropy as the likely source of enhanced thermopower in $\rm\bf Na_xCo_2O_4

In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This Peltier effect underlies all thermoelectric refrigerators. The upsurge in thermoelectric cooling applications has led to a search for more efficient Peltier materials and to renewed theoretical interest in how electron-electron interaction may enhance the thermopower $Q$ of materials such as the transition-metal oxides \cite{Mahan,Beni,Kotliar,Chaikin}. An important factor in this enhancement is the electronic spin entropy, which is predicted \cite{Chaikin,Kwak,KwakChaikin} to dominate the entropy current. Here we report evidence for such suppression in the layered oxide $\rm Na_xCo_2O_4$, and present evidence that it is a strong-correlation effect.

cond-mat.str-el↗
arXiv2004

Theory of Asymmetric Tunneling in the cuprate superconductors

We explain quantitatively, within the Gutzwiller-Resonating Valence Bond theory, the puzzling observation of tunneling conductivity between a metallic point and a cuprate high-$T_c$ superconductor which is markedly asymmetric between positive and negative voltage biases. The asymmetric part does not have a "coherence peak" but does show structure due to the gap. The fit to data is satisfactory within the over-simplifications of the theory; in particular, it explains the marked "peak-dip-hump" structure observed on the hole side and a number of other qualitative observations. This asymmetry is strong evidence for the projective nature of the ground state and hence for "t-J" physics.

cond-mat.supr-con↗
arXiv2004

Dissipationless Anomalous Hall Current in the Ferromagnetic Spinel CuCr$_2$Se$_{4-x}$Br$_x$

In a ferromagnet, an applied electric field $\bf E$ invariably produces an anomalous Hall current ${\bf J}_H$ that flows perpendicular to the plane defined by $\bf E$ and $\bf M$ (the magnetization). For decades, the question whether ${\bf J}_H$ is dissipationless (independent of the scattering rate), has been keenly debated without experimental resolution. In the ferromagnetic spinel CuCr$_2$Se$_{4-x}$Br$_x$, the resistivity $ρ$ (at low temperature) may be increased 1000 fold by varying $x$(Br), without degrading the $\bf M$. We show that ${\bf J}_H/E$ (normalized per carrier, at 5 K) remains unchanged throughout. In addition to resolving the controversy experimentally, our finding has strong bearing on the generation and study of spin-Hall currents in bulk samples.

cond-mat.str-el↗
arXiv2004

Low temperature phase transitions and crystal structure of Na0.5CoO2

The crystal structure of Na0.5CoO2, determined by powder neutron diffraction, is reported. The structure consists of layers of edge-shared CoO6 octahedra in a triangular lattice, with Na ions occupying ordered positions in the interleaving planes. The Na ions form one-dimensional zigzag chains. Two types of Co ions, which differ only subtly in their coordination by oxygen, are also found in chains. Specific heat measurements show that the transitions observed at 87 K and 53 K in the resistivity and magnetic susceptibility are accompanied by changes in entropy. Electron diffraction studies suggest that the 87K transition may have a structural component.

cond-mat.str-el↗
arXiv2004

Charge-ordering, commensurability and metallicity in the phase diagram of layered Na(x)CoO(2)

The phase diagram of non-hydrated Na(x)CoO(2) has been determined by changing the Na content x using a series of chemical reactions. As x increases from 0.3, the ground state goes from a paramagnetic metal to a charge-ordered insulator (at x=1/2) to a `Curie-Weiss metal' (around 0.70), and finally to a weak-moment magnetically ordered state (x>0.75). The unusual properties of the state at 1/2 (including particle-hole symmetry at low T and enhanced thermal conductivity) are described. The strong coupling between the Na ions and the holes is emphasized.

cond-mat.supr-con↗
arXiv2004

Phase coherence and the Nernst effect at magic angles in organic conductors

A giant Nernst signal was recently observed for fields near crystallographic directions in (TMTSF)$_2$PF$_6$. Such large Nernst signals are most naturally associated with the motion of pancake vortices. We propose a model in which phase coherence is destroyed throughout the sample except in planes closely aligned with the applied field $\bf H$. A small tilt above or below the plane changes the direction and density of the penetrating vortices and leads to a Nernst signal that varies with the tilt angle of $\bf H$ as observed. The resistance notches at magic angles are understood in terms of flux-flow dissipation from field-induced vortices.

cond-mat.supr-con↗
arXiv2003

Vorticity and the Nernst Effect in Cuprate Superconductors

We present a review of the vortex-Nernst effect in the 3 cuprate families La_{2-x}Sr_xCuO_4, Bi_2Sr_2CaCu_2O_8, and YBa_2Cu_3O_{y}, and discuss the scenario that the superconducting transition in the hole-doped cuprates corresponds to the destruction of long-range phase coherence rather than the vanishing of the order-parameter amplitude.

cond-mat.supr-con↗
arXiv2003

Vorticity, phase stiffness and the cuprate phase diagram

We review results obtained from vortex-Nernst experiments in cuprates. Evidence for a loss of phase coherence at the Meissner transition $T_{c0}$ is derived from vortex-like excitations that persist to high temperature. Below $T_{c0}$, the Nersnt signal provides a determination of the upper critical field $H_{c2}$ vs. doping $x$. Implications for the cuprate phase diagram are discussed.

cond-mat.supr-con↗
arXiv2003

Field-Tuning of the electron and hole populations in the ruthenate Bi_3Ru_3O_11

Experiments on the Hall coefficient R_H and heat capactity C reveal an unusual, compensated electronic ground state in the ruthenate Bi_3Ru_3O_11. At low temperature T, R_H decreases linearly with magnetic field |H| for fields larger than the field scale set by the Zeeman energy. The results suggest that the electron and hole populations are tuned by H in opposite directions via coupling of the spins to the field. As T is decreased below 5 K, the curve C(T)/T vs. T^2 shows an anomalous flattening consistent with a rapidly growing Sommerfeld parameter γ(T). We discuss shifts of the electron and hole chemical potentials by H to interpret the observed behavior of R_H.

cond-mat.str-el↗