The ground state of superconductors is characterized by the long-range order of condensed Cooper pairs: this is the only order present in conventional superconductors. The high-transition-temperature (high-T(c)) superconductors, in contrast, exhibit more complex phase behaviour, which might indicate the presence of other competing ground states. For example, the pseudogap--a suppression of the accessible electronic states at the Fermi level in the normal state of high-T(c) superconductors-has been interpreted as either a precursor to superconductivity or as tracer of a nearby ground state that can be separated from the superconducting state by a quantum critical point. Here we report the existence of a second order parameter hidden within the superconducting phase of the underdoped (electron-doped) high-T(c) superconductor Pr2-xCe(x)CuO4-y and the newly synthesized electron-doped material La2-xCe(x)CuO4-y (ref. 8). The existence of a pseudogap when superconductivity is suppressed excludes precursor superconductivity as its origin. Our observation is consistent with the presence of a (quantum) phase transition at T = 0, which may be a key to understanding high-T(c) superconductivity. This supports the picture that the physics of high-T(c) superconductors is determined by the interplay between competing and coexisting ground states.

We report on the preparation and characterization of the double-perovskite compound Sr2CrWO6 with a Curie temperature of 390 K. We have fabricated both Sr2CrWO6 bulk sintered polycrystalline bars and high-quality epitaxial thin films on SrTiO3 substrates by pulsed-laser deposition. The samples were characterized by thermogravimetric analysis, x-ray diffraction, electrical transport, and magnetization measurements. Polycrystalline samples containing a large number of grain boundaries show a large low-field magnetoresistance of up to 100% at 5 K. At room temperature, this effect is reduced to a few percent. Our results show that Sr2CrWO6 is an interesting candidate for room-temperature magnetoelectronic materials.

The use of oxide materials in oxide electronics requires their controlled epitaxial growth. Recently, it was shown that Reflection High Energy Electron Diffraction (RHEED) allows to monitor the growth of oxide thin films even at high oxygen pressure. Here, we report the sub-unit cell molecular or block layer growth of the oxide materials Sr 2 RuO 4 , MgO, and magnetite using Pulsed Laser Deposition (PLD) from stoichiometric targets. Whereas for perovskites such as SrTiO 3 or doped LaMnO 3 a single RHEED intensity oscillation is found to correspond to the growth of a single unit cell, in materials where the unit cell is composed of several molecular layers or blocks with identical stoichiometry, a sub-unit cell molecular or block layer growth is established resulting in several RHEED intensity oscillations during the growth of a single unit-cell.PACS numbers: 61.14. Hg, 74.76.Db, 81.15.Fg The physical properties of thin films are strongly influenced by their microstructure and morphology which, in turn, are determined by the deposition conditions and the growth mode. RHEED has proven to be a useful surface sensitive tool for monitoring in situ the growth of semiconductor thin films [1]. RHEED has also been successfully used for studying the growth of the complex copper oxide superconductors under high vacuum conditions [2,3,4]. Recently, high pressure RHEED systems have been developed allowing for the analysis of oxide thin films grown by Pulsed Laser Deposition (PLD) at high oxygen partial pressure of up to several 10 Pa [5,6]. A common way of using RHEED in the analysis of thin film growth is the observation of the electron diffraction pattern during epitaxial growth. In this case intensity oscillations of the diffraction spots are associated with a layer-by-layer or Frank-van der Merwe growth mode [2]. The deposited material nucleates on the substrate surface forming two dimensional islands which are coalescing with increasing coverage. This process results in a periodic roughening and flattening of the film surface translating into RHEED intensity oscillations.In this Letter, we report on the study of RHEED intensity oscillations during the PLD growth of the oxides materials Sr 2 RuO 4 , MgO, and magnetite (Fe 3 O 4 ) from stoichiometric targets. We have used an ultra high vacuum Laser Molecular Beam Epitaxy (L-MBE) system with in-situ high pressure RHEED and a 248 nm KrF excimer laser [7]. It is well known that due to the ionic bond character in oxides the different atomic layers in general will not be charge neutral and therefore the energetically most favorable growth unit often is a molecular layer which is composed of one or several atomic layers to obtain charge neutrality. Therefore, molecular layer or block layer epitaxy [7] is established for most oxides, whereas atomic layer epitaxy is common for semiconductor superlattice growth. Our results clearly show that under suitable deposition conditions a molecular or block layer growth is indeed established for MgO, Sr 2 RuO 4 and Fe 3 O 4 . However, i...

We report on the preparation and characterization of epitaxial thin films of the double-perovskite Sr 2 CrWO 6 by Pulsed Laser Deposition (PLD). On substrates with low lattice mismatch like SrTiO 3 , epitaxial Sr 2 CrWO 6 films with high crystalline quality can be grown in a molecular layer-by-layer growth mode. Due to the similar ionic radii of Cr and W, these elements show no sublattice order. Nevertheless, the measured Curie temperature is well above 400 K. Due to the reducing growth atmosphere required for double perovskites, the SrTiO 3 substrate surface undergoes an insulator-metal transition impeding the separation of thin film and substrate electric transport properties. [5]. Thin films of the well studied system Sr 2 FeMoO 6 have been fabricated by pulsed-laser deposition (PLD) at relatively high temperatures of about 900 • C [6]. However, epitaxial growth was found to be complicated and difficult to control. There are indications that high structural quality of films grown on SrTiO 3 is associated with semiconducting behavior [7,8]. Here, we report on the epitaxial growth of Sr 2 CrWO 6 . Due to the good lattice match, epitaxial films of this material can be grown on SrTiO 3 substrates in a molecular layer-by-layer growth mode resulting in high crystalline quality.Sr 2 CrWO 6 thin films were deposited from stoichiometric targets [5] on atomically flat, HF etched and annealed (001) SrTiO 3 substrates by PLD using a 248 nm KrF excimer laser [9]. Fig. 1 shows the RHEED (Reflection High Energy Electron Diffraction) [10] intensity oscillations of the (0, 0) diffraction spot recorded during the PLD growth of a c-axis oriented Sr 2 CrWO 6 film. The molecular layer-bylayer or Frank-van der Merwe growth mode [11] is achieved for a substrate temperature of 740 • C, an argon pressure of 2 × 10 −4 Torr, a laser repetition rate of 2 Hz, and a laser energy density on the target of 1.2 J/cm 2 . RHEED was performed with 15 keV electrons at an incident angle of about 2 • . To obtain the number of RHEED oscillations per unit cell, the film thickness has been determined precisely by X-ray reflectometry (Fig. 4) and then divided by the number of observed RHEED oscillations. The derived molecular layer or block thickness corresponds to half a unit cell (c/2 = 4.004Å). Note that the intensity oscillations of the (0,0) and (0,1) spot are out of phase, since for the (0,1) spot the electrons reflected from different growth planes interfere constructively (in-Bragg condition). In this case no RHEED oscillations are expected. The fact that we do observe RHEED oscillations is caused by multiple and diffuse, incoherent scattering which results in an * Electronic address: Boris.Philipp@wmi.badw.de increasing (decreasing) intensity with increasing (decreasing) step density [12,13]. Figure 2 shows an in situ AFM picture of a 42 nm thick film. Clearly, a terrace structure due to the nonvanishing substrate miscut with about 4Å high steps corresponding to half a unit cell of Sr 2 CrWO 6 can be seen. The AFM analysis shows that Sr 2 CrW...

Epitaxy of oxide materials on silicon (Si) substrates is of great interest for future functional devices using the large variety of physical properties of the oxides as ferroelectricity, ferromagnetism, or superconductivity. Recently, materials with high spin polarization of the charge carriers have become interesting for semiconductor-oxide hybrid devices in spin electronics. Here, we report on pulsed laser deposition of magnetite (Fe3O4) on Si(001) substrates cleaned by an in situ laser beam high temperature treatment. After depositing a double buffer layer of titanium nitride (TiN) and magnesium oxide (MgO), a high quality epitaxial magnetite layer can be grown as verified by RHEED intensity oscillations and high resolution x-ray diffraction.