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

Transport properties of polycrystalline Ge clathrates with general composition Sr8Ga16Ge30 are reported in the temperature range 5 K⩽T⩽300 K. These compounds exhibit N-type semiconducting behavior with relatively high Seebeck coefficients and electrical conductivity, and room temperature carrier concentrations in the range of 1017–1018 cm−3. The thermal conductivity is more than an order of magnitude smaller than that of crystalline germanium and has a glasslike temperature dependence. The resulting thermoelectric figure of merit, ZT, at room temperature for the present samples is 14 that of Bi2Te3 alloys currently used in devices for thermoelectric cooling. Extrapolating our measurements to above room temperature, we estimate that ZT>1 at T>700 K, thus exceeding that of most known materials.

/pdf/semiconducting-ge-clathrates-promising-candidates-for-wehk84knp6.pdf

Semiconducting Ge clathrates: Promising candidates for thermoelectric applications

▪ Abstract Jahn-Teller phenomena encountered in oxides are reviewed briefly. High-spin Mn(III) in an octahedral site illustrates an ion with twofold orbitally degenerate σ-bonding orbitals having a quenched orbital angular momentum; octahedral V(III) and high-spin Co(II), as well as tetrahedral high-spin Ni(II), are examples of ions with threefold orbitally degenerate π-bonding orbitals that retain orbital angular momentum. Cooperative local deformations that are static give rise, below a transition temperature Tt, to a lowering of the lattice symmetry, anisotropic interactions, and the possibility of a ferromagnetism that is oriented in opposition to a magnetic field applied on cooling through the transition. Dynamic local deformations can influence the physical properties of a solid in a variety of ways. Of special interest is the influence of dynamic deformations at a cross-over from localized to itinerant electronic behavior in a mixed-valent system.

Jahn-teller phenomena in solids

Charge- and spin-density waves in existing superconductors: competition between Cooper pairing and Peierls or excitonic instabilities

This review deals with the properties of superconductors with competing electron spectrum instabilities, namely, charge-density waves (CDWs) and spin-density waves (SDWs). The underlying reasons of the electron spectrum instability may be either Fermi surface nesting or the existence of Van Hove saddle points for lower dimensionalities. CDW superconductors include layered dichalcogenides, NbSe3, and compounds with the A15 and C15 structures among others. There is much evidence to show that high-Tc oxides may also belong to this group of materials. The SDW superconductors include URu2Si2 and related heavy-fermion compounds, Cr-Re alloys and organic superconductors. We review the experimental evidence for CDW and SDW instabilities in a wide range of different superconductors, and assess the competition between these instabilities of the Fermi surface and the superconducting gap. Issues concerning the superconducting order parameter symmetry are also touched upon. The accent is put on establishing a universal framework for further theoretical discussions and experimental investigations based on an extensive list of available and up-to-date references.

http://iopscience.iop.org/article/10.1088/0953-2048/14/4/201/pdf

Charge- and spin-density-wave superconductors

Measurements of thermal conductivity versus temperature and magnetic field are reported for perovskite manganites that exhibit ferromagnetic (FM), charge-ordering (CO), antiferromagnetic, and/or structural phase transitions. The data reveal a dominant lattice contribution to the heat conductivity with $\ensuremath{\kappa}\ensuremath{\sim}1\ensuremath{-}2$ W/mK near room temperature. The rather low values, implying a phonon mean free path on the order of a lattice spacing, are shown to correlate with static local distortions of the ${\mathrm{MnO}}_{6}$ octahedra. Modifications of the local structure are responsible for abrupt anomalies in the zero-field \ensuremath{\kappa} at the FM, CO, and structural transitions, and for colossal magnetothermal resistance near the FM transition.

http://web2.physics.miami.edu/~cohn/kmanganites.pdf

Local lattice distortions and thermal transport in perovskite manganites

Measurements of thermal conductivity versus temperature over a broad range of doping in YBa$_2$Cu$_3$O$_{6+x}$ and HgBa$_2$Ca$_{n-1}$Cu$_n$O$_{2n+2+\delta}$ ($n$=1,2,3) suggest that small domains of localized holes develop for hole concentrations near $p$=1/8. The data imply a mechanism for localization that is intrinsic to the CuO$_2$-planes and is enhanced via pinning associated with oxygen-vacancy clusters.

Hole localization in underdoped superconducting cuprates near (formula presented) doping

A systematic study of the temperature and oxygen-doping dependence of thermal conductivity ({kappa}) and thermoelectric power (S) in polycrystal YBa{sub 2}Cu{sub 3}O{sub 6+x} is reported for compositions spanning the phase diagram (0{lt}x{lt}1). {kappa}(x) is nonmonotonic with apparent enhancements near x=0.1, 0.3, and 0.6. Thermal hysteresis in {kappa} and S with a common temperature dependence is observed for all x, and is substantially enhanced at these same compositions. These features are likely related to the oxygen order and associated changes in the local structure. The superconducting-state enhancement of {kappa} is parametrized by its change in slope at the superconducting transition. The doping dependence of this quantity agrees with that of the in-plane thermal conductivity in single crystals, and confirms that the enhancement scales with the superconducting condensate density throughout the underdoped regime. {copyright} {ital 1997} {ital The American Physical Society}

C. P. Popoviciu

Papers

Local lattice distortions and thermal transport in perovskite manganites

Hole localization in underdoped superconducting cuprates near (formula presented) doping

Thermal transport in YBa2Cu3O6+x: Doping dependence across the phase diagram