Enhancement of the Seebeck coefficient (S ) without reducing the electrical conductivity (sigma) is essential to realize practical thermoelectric materials exhibiting a dimensionless figure of merit (ZT=S2 x sigma x T x kappa-1) exceeding 2, where T is the absolute temperature and kappa is the thermal conductivity. Here, we demonstrate that a high-density two-dimensional electron gas (2DEG) confined within a unit cell layer thickness in SrTiO(3) yields unusually large |S|, approximately five times larger than that of SrTiO(3) bulks, while maintaining a high sigma2DEG. In the best case, we observe |S|=850 microV K-1 and sigma2DEG=1.4 x 10(3) S cm-1. In addition, by using the kappa of bulk single-crystal SrTiO(3) at room temperature, we estimate ZT approximately 2.4 for the 2DEG, corresponding to ZT approximately 0.24 for a complete device having the 2DEG as the active region. The present approach using a 2DEG provides a new route to realize practical thermoelectric materials without the use of toxic heavy elements.

Giant thermoelectric Seebeck coefficient of a two-dimensional electron gas in SrTiO3

Raman spectra of titanium dioxide

The structure of SrTiO3 has been the subject of X-ray, [1] infrared, [2] ESR, [3–5] neutron, [6] and Raman [7–10] spectroscopic investigations since 1962. While the high-temperature phase is acknowledged to be simple cubic perovskite 0 h 1 , with one formula group per unit cell, there have been, until recently, apparent inconsistencies in the diverse data and a lack of agreement concerning the symmetry of the crystal at low temperatures. Following discovery of anomalies in the sound velocity of SrTiO3 near 110°K [11], ESR studies [3] showed the presence of a cubic to tetragonal phase transition. This was verified by means of X-ray analysis [1]; in the latter study the c/a ratio was determined to be 1.000 56, and two other phase transitions were inferred to be at 65 and 35°K. Infrared and neutron studies [2, 6] supported the X-ray symmetry assignment of structure at 77 °K: the crystal was viewed as having domain structure with one of the equivalent (100) axes in each domain slightly elongated in the tetragonal phase; below 110°K the crystal was thought to retain a single formula group per unit cell and all atoms at inversion centers. The distortion from cubic symmetry was apparently so slight, in fact, that even the predicted F1u → Eu + A2u doubling below 110°K of each of the Oh -phase IR-active vibrations was unobservable in infrared studies. [2]

Soft Phonon Modes and the 110°K Phase Transition in SrTiO3

The permittivity of single‐crystal single‐domain strontiumtitanate has been measured in detail in the [001], [011], and [111] directions, as a function of temperature (from 4.2 to 300 °K), electric field (from −23 000 to +23 000 V/cm, and frequency (from 1 kHz to 50 MHz). The free energy of the crystal is determined as a function of polarization with temperature as a parameter. The Curie‐Weiss law is satisfied in the temperature range 60–300 °K, giving a Curie temperature of 30 ± 2 °K for the three crystal orientations. The Lyddane‐Sachs‐Teller (LST) relation is satisfied for temperatures between 30 and 300 °K and for electric fields between 0 and 12 000 V/cm. A generalized LST relation is used to calculate the permittivity of strontiumtitanate from zero to optic frequencies. Two active optic modes are important. The lower‐frequency mode is attributed mainly to motion of the strontium ions with respect to the rest of the lattice, while the higher‐frequency active mode is attributed to motion of the titanium ions with respect to the oxygen lattice. The restoring forces that act on the Ti ions begin to "harden" when these ions are displaced approximately 0.002 A from their equilibrium positions.

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Permittivity of Strontium Titanate

We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature (Tc) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO3 layers in BaTiO3/SrTiO3 superlattices are not only ferroelectric (with Tc as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO3 layers adjacent to them. Tc was tuned by approximately 500 kelvin by varying the thicknesses of the BaTiO3 and SrTiO3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity.

/pdf/probing-nanoscale-ferroelectricity-by-ultraviolet-raman-1zt7op2esk.pdf

Probing Nanoscale Ferroelectricity by Ultraviolet Raman Spectroscopy

The Raman spectrum of single‐crystal SrTiO3 has been investigated as a function of temperature down to about 25°K. The lattice dynamics of this crystal have been extensively studied in recent years primarily because of its unusual dielectric and acoustic properties. Both nominally pure and impure samples were examined since these properties seem to be sensitive to small amounts of impurities. An argon‐ion laser served as the exciting source and an in‐tandem grating spectrometer was used to disperse the scattered light. These instrumental techniques provided rather complete data including the polarization characteristics of the individual lines and peaks. The above data, together with the known phonon dispersion curves of SrTiO3, made it possible to obtain a reliable interpretation of the Raman spectrum. At room temperature we find the Raman spectrum to be entirely second order, in agreement with the selection rules for the cubic perovskite structure. Further, the phonons contributing to the second‐order s...

W. G. Nilsen

Papers

Raman Spectrum of Strontium Titanate