No abstract

No abstract

Application of the field-effect transistor principle to novel materials to achieve electrostatic doping is a relatively new research area. It may provide the opportunity to bring about modifications of the electronic and magnetic properties of materials through controlled and reversible changes of the carrier concentration without modifying the level of disorder, as occurs when chemical composition is altered. As well as providing a basis for new devices, electrostatic doping can in principle serve as a tool for studying quantum critical behavior, by permitting the ground state of a system to be tuned in a controlled fashion. In this paper progress in electrostatic doping of a number of materials systems is reviewed. These include structures containing complex oxides, such as cuprate superconductors and colossal magnetoresistive compounds, organic semiconductors, in the form of both single crystals and REVIEWS OF MODERN PHYSICS, VOLUME 78, OCTOBER-DECEMBER 20060034-6861/2006/78͑4͒/1185͑28͒ ©2006 The American Physical Society 1185 thin films, inorganic layered compounds, single molecules, and magnetic semiconductors. Recent progress in the field is discussed, including enabling experiments and technologies, open scientific issues and challenges, and future research opportunities. For many of the materials considered, some of the results can be anticipated by combining knowledge of macroscopic or bulk properties and the understanding of the field-effect configuration developed during the course of the evolution of conventional microelectronics. However, because electrostatic doping is an interfacial phenomenon, which is largely an unexplored field, real progress will depend on the development of a better understanding of lattice distortion and charge transfer at interfaces in these systems.

An angle-resolved photoemission study is reported on Ca 2 CuO 2 Cl 2 , a parent compound of high- T c superconductors. Analysis of the electron occupation probability, n( k ), from the spectra shows a steep drop in spectral intensity across a contour that is close to the Fermi surface predicted by the band calculation. This analysis reveals a Fermi surface remnant, even though Ca 2 CuO 2 Cl 2 is a Mott insulator. The lowest energy peak exhibits a dispersion with approximately the |cos k x a – cos k y a | form along this remnant Fermi surface. Together with the data from Dy-doped Bi 2 Sr 2 CaCu 2 O 8+δ , these results suggest that this d -wave–like dispersion of the insulator is the underlying reason for the pseudo gap in the underdoped regime.

We report that the doping and temperature dependence of photoemission spectra near the Brillouin zone boundary of Bi 2 Sr 2 CaCu 2 O 8+δ δ exhibit unexpected sensitivity to the superfluid density. In the superconducting state, the photoemission peak intensity as a function of doping scales with the superfluid density and the condensation energy. As a function of temperature, the peak intensity shows an abrupt behavior near the superconducting phase transition temperature where phase coherence sets in, rather than near the temperature where the gap opens. This anomalous manifestation of collective effects in single-particle spectroscopy raises important questions concerning the mechanism of high-temperature superconductivity.The collective nature of superconductivity manifests itself contrastingly in different techniques. Microwave and muon spin relaxation measurements are inherently sensitive to the collective motion of the condensate, whereas single-electron tunneling spectroscopy and photoemission mainly probe single-particle excitations of the condensate. Hence, these two types of spectroscopies can be used to measure two essential but distinct ingredients of superconductivity: the superfluid density, which characterizes the phase coherence of the Cooper pairs, and the superconducting energy gap, which reflects the strength of the pairing. We report a pronounced departure from

Emergent phenomena, including superconductivity and magnetism, found in the two-dimensional electron liquid (2-DEL) at the interface between the insulators lanthanum aluminate (LaAlO 3 ) and strontium titanate (SrTiO 3 ) distinguish this rich system from conventional 2D electron gases at compound semiconductor interfaces. The origin of this 2-DEL, however, is highly debated, with focus on the role of defects in the SrTiO 3 , while the LaAlO 3 has been assumed perfect. Here we demonstrate, through experiments and firstprinciple calculations, that the cation stoichiometry of the nominal LaAlO 3 layer is key to 2-DEL formation: only Al-rich LaAlO 3 results in a 2-DEL. Although extrinsic defects, including oxygen deficiency, are known to render LaAlO 3 /SrTiO 3 samples conducting, our results show that in the absence of such extrinsic defects an interface 2-DEL can form. Its origin is consistent with an intrinsic electronic reconstruction occurring to counteract a polarization catastrophe. This work provides insight for identifying other interfaces where emergent behaviours await discovery.

Abstract:Superlattices of (LaMnO 3 ) 2n /(SrMnO 3 ) n (1≤ n ≤ 5), composed of the insulators LaMnO 3 and SrMnO 3 , undergo a metal-insulator transition as a function of n, being metallic for n≤2 and insulating for n≥3. Measurements of transport, magnetization and polarized neutron reflectivity reveal that the ferromagnetism is relatively uniform in the metallic state, and is strongly modulated in the insulating state, being high in LaMnO 3 and suppressed in SrMnO 3 . The modulation is consistent with a Mott transition driven by the proximity between the (LaMnO 3 )/(SrMnO 3 ) interfaces. Disorder localizes states at the Fermi level at the interfaces for n ≥ 3. We suggest that this disorder is due to magnetic frustration at the interfaces.

We use resonant soft-x-ray scattering (RSXS) to study the electronic reconstruction at the interface between the Mott insulator LaMnO3 and the band insulator SrMnO3. Superlattices of these two insulators were shown previously to have both ferromagnetism and metallic tendencies [Koida, Phys. Rev. B 66, 144418 (2002)10.1103/PhysRevB.66.144418]. By studying a judiciously chosen superlattice reflection, we show that the interface density of states exhibits a pronounced peak at the Fermi level, similar to that predicted in related titanate superlattices by Okamoto et al. [Phys. Rev. B 70, 241104(R) (2004)10.1103/PhysRevB.70.241104]. The intensity of this peak correlates with the conductivity and magnetization, suggesting it is the driver of metallic behavior. Our study demonstrates a general strategy for using RSXS to probe the electronic properties of heterostructure interfaces.