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

Distortions of the electronic density of states (DOS) are a potent mechanism to increase the thermopower of thermoelectric semiconductors, thereby increasing their power factor. We review band-structure engineering approaches that have been used to achieve this, resonant impurity levels, dilute Kondo effects, and hybridization effects in strongly correlated electron systems. These can increase the thermoelectric power of metals and semiconductors through two mechanisms: (1) the added density of states increases the thermopower in a nearly temperature-independent way; (2) resonant scattering results in a strong electron energy filtering effect that increases the thermopower at cryogenic temperatures where the electron–phonon interactions are weaker. Electronic structure calculation results for Tl:PbTe and Ti:PbTe are contrasted and identify the origin of the thermopower enhancement in Tl:PbTe. This leads to a discussion of the conditions for DOS distortions to produce thermopower enhancements and illustrates the existence of an optimal degree of delocalization of the impurity states. The experimentally observed resonant levels in several III–V, II–VI, IV–VI and V2-VI3 compound semiconductor systems are reviewed.

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Resonant levels in bulk thermoelectric semiconductors

Defects and defect engineering are at the core of many regimes of material research, including the field of thermoelectric study. The 60-year history of V2VI3 thermoelectric materials is a prime example of how a class of semiconductor material, considered mature several times, can be rejuvenated by better understanding and manipulation of defects. This review aims to provide a systematic account of the underexplored intrinsic point defects in V2VI3 compounds, with regard to (i) their formation and control, and (ii) their interplay with other types of defects towards higher thermoelectric performance. We herein present a convincing case that intrinsic point defects can be actively controlled by extrinsic doping and also via compositional, mechanical, and thermal control at various stages of material synthesis. An up-to-date understanding of intrinsic point defects in V2VI3 compounds is summarized in a (χ, r)-model and applied to elucidating the donor-like effect. These new insights not only enable more innovative defect engineering in other thermoelectric materials but also, in a broad context, contribute to rational defect design in advanced functional materials at large.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/advs.201600004

New Insights into Intrinsic Point Defects in V2VI3 Thermoelectric Materials

Misfit layer compounds : Structures and physical properties

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

A new technique is described in which a transient thermoelectric (TTE) voltage of a semiconductor is measured after a pulsed laser irradiation—a modification of photodiffusion or Dember effect. This technique has been successfully applied to test samples of n‐ and p‐Si over the wide time range from 20 ns to 2 s and the temperature 120–284 K. The decay curves of the TTE voltage consist of three stages with the characteristic relaxation or decay times. Stage 1 with the relaxation time τ1, is associated with the carrier generation and recombination and explained by the Dember effect. Stage 2 characterized by the relaxation time τ2, is due to the diffusion of majority carriers from a higher to a lower temperature region; for p‐Si, a double‐relaxation process is observed arising from the difference in the drift mobilities of light and heavy holes. At stage 3, the TTE voltage vanishes completely, which can be reasonably interpreted by the diffusion of thermal flux or phonons along a temperature gradient produced by the laser pulse. Analytical expressions for each stage are presented to discuss these experimental results.

Pulsed laser‐induced transient thermoelectric effects in silicon crystals

Mesure des parametres a et c des composes M x TiS 2 obtenus par transport chimique sur le domaine de concentration 0≤x≤1

Interlayer spacing of 3d transition-metal intercalates of 1T-cadmium iodide-type titanium disulfide (TiS2)

Specific heats of 3d transition metal intercalates of 1T-CdI2-type TiS2, M
 x
 TiS2 (M=V, Cr, Mn, Fe, Co, and Ni; 0≤x≤1), have been measured in the temperature range 1.6–300 K using an ac calorimetry technique. The electronic specific heat coefficient γ (2–100 mJ/mole K2) and the Debye temperature ϑD (240–430 K) are found to depend on the guest 3d metals and their concentrations. All the intercalates show anomalous specific heat at low temperatures following an α–δ lnT dependence (α and δ are constants), as found in dilute alloys.

Specific heat measurements of intercalation compounds M x TiS 2 (M=3 d transition metals) using ac calorimetry technique

The temperature dependence of electrical resistivity and thermoelectric power of the intercalation compounds of MxTiS2 (M = Mn, Fe, Co, and Ni; 0 ≦ x ≦ 1/3) with layered structures are measured over the temperature range 1.5 to 300 K. These experimental results can be explained by a simple model, which takes into account a temperature-independent residual term and intra- and intervalley scattering by acoustic phonons. It is also shown that a phonon drag effect is effective for the thermopower. The effect of intercalation of 3d metals into TiS2 layers on the electronic properties is discussed.



Die Temperaturabhangigkeit des elektrischen Widerstands und der Thermospannung der Interkalate von MxTiS2 (M = Mn, Fe, Co, und Ni; 0 ≦ x ≦ 1/3) mit Schichtstrukturen werden bei Temperaturen von 1,5 bis 300 K gemessen. Die experimentellen Ergebnisse konnen durch ein einfaches Modell erklart werden, das einen temperaturunabhangigen Restterm sowie „intra- und intervalley scattering” durch akustische Phononen berucksichtigt. Es wird auch gezeigt, das ein „phonon-drag”-Effekt fur die Thermospannung wirksam ist. Der Einflus der Interkalation von 3d-Metallen in die TiS2-Schichten auf die elektronischen Eigenschaften wird diskutiert.

Masasi Inoue

Papers

Pulsed laser‐induced transient thermoelectric effects in silicon crystals

Interlayer spacing of 3d transition-metal intercalates of 1T-cadmium iodide-type titanium disulfide (TiS2)

Specific heat measurements of intercalation compounds M x TiS 2 (M=3 d transition metals) using ac calorimetry technique

Electrical Resistivity and Thermoelectric Power of Intercalation Compounds MxTiS2 (M = Mn, Fe, Co, and Ni)

Resonant photoemission studies of 3d transition metal intercalates of TiS2