This article discusses fluctuating order in a quantum disordered phase proximate to a quantum critical point, with particular emphasis on fluctuating stripe order. Optimal strategies are derived for extracting information concerning such local order from experiments, with emphasis on neutron scattering and scanning tunneling microscopy. These ideas are tested by application to two model systems---an exactly solvable one-dimensional (1D) electron gas with an impurity, and a weakly interacting 2D electron gas. Experiments on the cuprate high-temperature superconductors which can be analyzed using these strategies are extensively reviewed. The authors adduce evidence that stripe correlations are widespread in the cuprates. They compare and contrast the advantages of two limiting perspectives on the high-temperature superconductor: weak coupling, in which correlation effects are treated as a perturbation on an underlying metallic (although renormalized) Fermi-liquid state, and strong coupling, in which the magnetism is associated with well-defined localized spins, and stripes are viewed as a form of micro phase separation. The authors present quantitative indicators that the latter view better accounts for the observed stripe phenomena in the cuprates.

How to detect fluctuating stripes in the high-temperature superconductors

Single crystals of (Ca2CoO3)0.7CoO2 were grown using a modified strontium chloride flux technique, and their electrical and thermal properties were determined. Growth conditions were established for obtaining plate-like single crystals of a relatively large size (5×10×0.05 mm3). At 973 K, thermoelectric power S and electrical resistivity ρ of the specimens are ≈240 μV K−1 and ≈2.3×10−5 Ω m, respectively, and their thermal conductivity κ is nearly 3 W m1 K−1, as determined by mathematical extrapolation. The figure of merit ZT=S2T/ρκ derived therefrom is ≈0.87 at 973 K. The relatively low κ is likely the result of a misfit structure between the CoO2 layer and the Ca2CoO3 slab.

Electrical and thermal properties of single-crystalline (Ca2CoO3)0.7CoO2 with a Ca3Co4O9 structure

Thermoelectric (TE) materials facilitate direct heat-to-electricity conversion. The performance of a TE material is characterised by its figure of merit zT (=S2 σT/κ) that depends on both electronic transport properties, i.e. the Seebeck coefficient S and the electrical conductivity σ, and on thermal transport properties, i.e. the thermal conductivity κ of a material. The intrinsically counter-correlated behaviour between electronic and thermal transport properties makes the enhancement of zT a very challenging task. In the past 10 years, the zTs in bulk TE materials have been significantly enhanced due to intensive exploratory efforts, the discovery of new physical phenomena and effects, and applications of advanced synthesis methods. In this review, we summarise the recent progress in bulk TE materials. After the introduction of fundamental principles of thermoelectricity, the recently achieved enhancements in the TE performance encompassing the use of electronic band structure engineering, lattice phon...

Recent advances in high-performance bulk thermoelectric materials

The extremely high thermal conductivities of carbon nanotubes have motivated a wealth of research. Progress includes innovative conduction metrology based on microfabricated platforms and scanning thermal probes as well as simulations exploring phonon dispersion and scattering using both transport theory and molecular dynamics. This article highlights these advancements as part of a detailed review of heat conduction research on both individual carbon nanotubes and nanostructured films consisting of arrays of nanotubes or disordered nanotube mats. Nanotube length, diameter, and chirality strongly influence the thermal conductivities of individual nanotubes and the transition from primarily diffusive to ballistic heat transport with decreasing temperature. A key experimental challenge, for both individual nanotubes and aligned films, is the separation of intrinsic and contact resistances. Molecular dynamics simulations have studied the impacts of specific types of imperfections on the nanotube conductance and its variation with length and chirality. While the properties of aligned films fall short of predictions based on individual nanotube data, improvements in surface engagement and postfabrication nanotube quality are promising for a variety of applications including mechanically compliant thermal contacts.

http://nanoheat.stanford.edu/sites/default/files/publications/RevModPhys%20Marconnet%20et%20al..pdf

Thermal conduction phenomena in carbon nanotubes and related nanostructured materials

We measure the high-temperature thermoelectric properties of the NaxCoO2-? single crystal for the first time. The NaxCoO2-? single crystals are prepared by a flux technique, and the resulting flaky single crystals are very thin on the c-axis. The in-plane electrical resistivity (?), the thermoelectric power (S) and the in-plane thermal conductivity (?) are measured in the range of 300 K to 800 K. The estimated power factor (PF=S2??-1) and the figure-of-merit (Z=S2??-1?-1) are about 2.4 mW m-1 K-2 and 0.12 mK-1 at 300 K, respectively. These factors increase with temperature, and reach the value of PF=7.7 mW m-1 K-2 and Z=1.5 mK-1 at 800 K, and the dimensionless value of ZT exceeds the criterion of 1. The PF and Z exceed the values of typical ?high-temperature thermoelectric materials?, such as p-type FeSi2 and Si0.7Ge0.3. These results suggest that NaxCoO2-? could be a very promising material for use in high-temperature thermoelectric power generation devices.

https://iopscience.iop.org/article/10.1143/JJAP.40.4644/pdf

High-Temperature Thermoelectric Properties of NaxCoO2-δ Single Crystals

Electric resistivity, thermoelectric power and thermal conductivity of a polycrystalline sample of the composite crystal [Ca2CoO3.34]0.614[CoO2], also known as Ca3Co4O9, have been measured below 300 K. Metallic conductivity accompanied by large thermoelectric power has been observed down to 50 K. At 300 K, the sample exhibits a thermoelectric power of S = 133 µVK-1, resistivity of ρ= 15 mΩcm and thermal conductivity of κ= 9.8 mWK-1cm-1. The resulting dimensionless figure of merit becomes ZT300 = 3.5×10-2, which is comparable to the value reported for a polycrystalline sample of NaCo2O4, indicating that the title compound is a potential candidate for a thermoelectric material.

https://iopscience.iop.org/article/10.1143/JJAP.39.L531/pdf

Low-Temperature Thermoelectric Properties of the Composite Crystal [Ca 2CoO 3.34] 0.614[CoO 2]

We have found an anomalous suppression of superconductivity at $x=0.30--0.35$, where $p$ (the hole concentration per Cu) $\ensuremath{\sim} 1/8$, in the partially Zn-substituted compound ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Y}}_{x}({\mathrm{Cu}}_{1\ensuremath{-}y}{\mathrm{Zn}}_{y}{)}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ with $y=0.02--0.03$. In these samples with $p\ensuremath{\sim}1/8$ and $y=0.02--0.03$, transport properties such as electrical resistivity and thermoelectric power exhibit less metallic behavior than usual. There is a possibility that a kind of order of holes and/or spins is stabilized owing to pinning by Zn, as in the La-based cuprate. It is likely that the so-called ``1/8 problem'' is not only characteristic of the La-based cuprate but also common to all high-${T}_{c}$ cuprates including ${\mathrm{CuO}}_{2}$ planes in their crystal structures.

Anomalous suppression of superconductivity in Zn-substituted Bi 2 Sr 2 Ca 1 − x Y x ( Cu 1 − y Zn y ) 2 O 8 + δ

The science and technology of carbon nanotubes are very interesting topics in the field of nanotechnology. It is considered that such nanotubes have excellent properties of electrical conduction, tensile strength, and thermal conduction. However, these properties are not well understood yet. Techniques for handling a nanotube and measuring these properties have not yet been established. Recently, a technique for synthesizing supergrowth carbon nanotubes, which form a highly pure single-walled carbon nanotube (SWNT) forest, has been developed. These supergrowth carbon nanotubes grow to as long as a millimeter scale. As-grown supergrowth carbon nanotubes include about 5×1011 SWNTs per square centimeter. The solid of supergrowth carbon nanotubes is prepared by reducing the distance between SWNTs. We have investigated the thermal conduction of such carbon nanotubes and were successful in measuring the thermal diffusivity of self-standing samples of as-grown supergrowth carbon nanotubes and their solid using the laser flash method. It was found that the carbon nanotube samples show a comparable thermal diffusivity to isotropic graphite. We have also measured the temperature dependence of the thermal diffusivity of the supergrowth carbon nanotube samples.

https://iopscience.iop.org/article/10.1143/JJAP.48.05EC07/pdf

Megumi Akoshima

Papers

Low-Temperature Thermoelectric Properties of the Composite Crystal [Ca 2CoO 3.34] 0.614[CoO 2]

Anomalous suppression of superconductivity in Zn-substituted Bi 2 Sr 2 Ca 1 − x Y x ( Cu 1 − y Zn y ) 2 O 8 + δ

Thermal Diffusivity of Single-Walled Carbon Nanotube Forest Measured by Laser Flash Method

On the 60 K plateau of Tc in Y1-xCaxBa2Cu3O7-δ in relation to the 1/8 problem

Anomalous muon-spin relaxation in the Zn-substituted YBa 2 Cu 3-2y Zn 2y O 7-δ around the hole concentration of 18 per Cu