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Optical Response of High-Dielectric-Constant Perovskite-Related Oxide
C. C. Homes,1* T. Vogt,1 S. M. Shapiro,1 S. Wakimoto,1,2† A. P. Ramirez3
Optical conductivity measurements on the perovskite-related oxide CaCu3Ti4O12 provide a hint of the physics underlying the observed giant dielectric effect in this material. A low-frequency vibration displays anomalous behavior, implying thatthere is a redistribution of charge within the unit cell at low temperature. At infrared frequencies (terahertz), the value for the dielectric constant is 80 at room temperature, which is far smaller than the value of 105 obtained at lower radio frequencies (kilohertz). This discrepancy implies the presence of a strong absorption at very low frequencies due to dipole relaxation. At roomtemperature, the characteristic relaxation times are fast ( 500 nanoseconds) but increase dramatically at low temperature, suggesting that the large change in dielectric constant may be due to a relaxor-like dynamical slowing down of dipolar fluctuations in nanosize domains. Microelectronics is driven by an almost insatiable appetite for smaller and faster devices. In memory devices based on capacitivecomponents, such as static and dynamic random access memories, the static dielectric constant ε0 of a material will ultimately decide the degree of miniaturization. Perovskites possess high values for ε0 and are widely used in such technological applications. The dielectric constant of a material is related to the polarizability , in particular the dipole polarizability (an atomic property), whicharises from structures with a permanent electric dipole that can change orientation in an applied electric field. These two quantities are linked through the Clausius-Mossotti relation. In metals, the charge is delocalized 0. In insulators, the charge is and ε0 localized and ε0 0; materials with a dielectric constant greater than that of silicon nitride (ε0 7) are classified as high-dielectricconstantmaterials. In general, a value of ε0 above 1000 is related to either a ferroelectric that exhibits a dipole moment in the absence of an external electric field or to a relaxor characterized by a ferroelectric response under high electric fields at lower temperature, but no macroscopic spontaneous polarization. However, both classes of materials show a
Department of Physics, Building 510B,Brookhaven National Laboratory, Upton, NY 11973–5000, USA. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139 – 4307, USA. 3 Condensed Matter and Thermal Physics Group, MST10 K764, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
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peak of ε0 as a function of temperature, which is undesirable for many applications. For instance, capacitors [with capacitanceC ε0 A/d; two plates of area A separated by a distance d] need to have static values to operate properly under a variety of conditions; if ε0 has a strong temperature dependence, then the device will not be robust and may fail. Recently, the perovskiterelated body-centered cubic (bcc) material CaCu3Ti4O12 (CCTO) revealed an extraordinarily high dielectric constant at room temperature of 105 (1,2) and was found to be practically constant between 100 and 600 K. Both properties are very important for device implementation (3, 4). However, ε0 displays a 1000-fold reduction below 100 K, without

Fig. 1. Several unit cells of CaCu3Ti4O12, shown as TiO6 octahedra, Cu atoms (blue) bonded to four oxygen atoms (red), and large Ca atoms ( yellow) without bonds.

*To whom correspondence shouldbe addressed. Email: homes@bnl.gov †Present address: Department of Physics, University of Toronto, 60 St. George Street, Toronto, Ontario, Canada, M5S 1A7.

www.sciencemag.org SCIENCE VOL 293 27 JULY 2001

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any detectable change of the long-range crystallographic structure when probed by highresolution x-ray (2) and neutron...