The competition between collective quantum phases in materials with strongly correlated electrons depends sensitively on the dimensionality of the electron system, which is difficult to control by standard solid-state chemistry. We have fabricated superlattices of the paramagnetic metal lanthanum nickelate (LaNiO(3)) and the wide-gap insulator lanthanum aluminate (LaAlO(3)) with atomically precise layer sequences. We used optical ellipsometry and low-energy muon spin rotation to show that superlattices with LaNiO(3) as thin as two unit cells undergo a sequence of collective metal-insulator and antiferromagnetic transitions as a function of decreasing temperature, whereas samples with thicker LaNiO(3) layers remain metallic and paramagnetic at all temperatures. Metal-oxide superlattices thus allow control of the dimensionality and collective phase behavior of correlated-electron systems.

The excitonic insulator is a long conjectured correlated electron phase of narrow-gap semiconductors and semimetals, driven by weakly screened electron–hole interactions. Having been proposed more than 50 years ago, conclusive experimental evidence for its existence remains elusive. Ta2NiSe5 is a narrow-gap semiconductor with a small one-electron bandgap EG of <50 meV. Below TC=326 K, a putative excitonic insulator is stabilized. Here we report an optical excitation gap Eop ∼0.16 eV below TC comparable to the estimated exciton binding energy EB. Specific heat measurements show the entropy associated with the transition being consistent with a primarily electronic origin. To further explore this physics, we map the TC–EG phase diagram tuning EG via chemical and physical pressure. The dome-like behaviour around EG∼0 combined with our transport, thermodynamic and optical results are fully consistent with an excitonic insulator phase in Ta2NiSe5.

arXiv:1101.0238v1 [cond-mat.str-el]

The distribution of valence electrons in metals usually follows the symmetry of an ionic lattice. Modulations of this distribution often occur when those electrons are not stable with respect to a new electronic order, such as spin or charge density waves. Electron Calculations of the electronic structure of the new pnictide superconductors unanimously predict a Fermi surface (FS) consisting of hole-like pocket in the centre (Γ point) of the Brillouin zone (BZ) and electron-like ones at the corners (X point) of the BZ. A shift by the (π, π) vector would result in a significant overlap of these FSs. Such an electronic structure is highly unstable since any interaction allowing an electron to gain a (π, π) momentum would favour a density wave order, which then results in aforementioned shift and a concomitant opening of the gaps, thus strongly reducing the electronic kinetic energy. It is surprising that ARPES data are reported to be in general, and sometimes in very detailed [9], agreement with the calculations giving a potentially unstable solution [5,6,7]. Even in the parent compound, where the spin density wave transition is clearly seen by other techniques [16,17], no evidence for the expected energy gap has been detected by photoemission experiments [7,8]. In fact, no consensus exists regarding the overall FS topology. According to Refs. 6 and 5, there is a single electron-like FS pocket around the X point, while Ref. 18 reports two intensity spots without any discernible signature for the electron pocket in the normal state. Intensity spots near the X point can also be found in Refs. 6, 7 and 9, but those are interpreted as parts of electron-like pockets. Obviously, such substantial variations in the photoemission signal preclude unambiguous assignment of the observed features to the calculated FS, leaving the electronic structure of the arsenides unclear.In Fig. 1 we show experimental FS map of Ba 1−x K x Fe 2 As 2 (BKFA) measured in superconducting state. To eliminate possible effects of photoemission matrix elements, as well as to cut the electronic structure at different k z values, we have done measurements at several excitation energies (Fig. 1a-b) and polarizations ( Fig. 1c-d). Although there are obvious changes in the intensities of the features, no signatures indicating k z dispersion can be concluded. With this in mind, the apparently different intensity distributions at neighboring Γ points appear unusual. While in the first BZ the two concentric contours are broadly consistent with