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Oxide Molecular Beam Epitaxy Group
Publications
2009
J. A. Clayhold, O. Pelleg, D. W. Rench, B. M. Kerns, M. D. Schroer, D. C. Ingram, A. T. Bollinger, G. Logvenov, R. J. Sundling, and I. Bozovic. “Constraints on Models of Electrical Transport in Optimally Doped La2-xSrxCuO4 from Measurements of Radiation-Induced Defect Resistance.” Journal of Superconductivity and Novel Magnetism (2009). In press.  Abstract
Precise measurements of the temperature dependence of additional resistivity caused by defect scattering
were used to constrain models of carrier transport in La1.84Sr0.16CuO4. Where previous magnetotransport
studies have delineated two distinct scattering processes, proportional to T and T2,
respectively, the new defect scattering results suggest strongly that the two processes act as
parallel conductance channels. | G. Logvenov, A. Gozar, V. Y. Butko, A. T. Bollinger, N. Bozovic, Z. Radovic, and I. Bozovic. “Comprehensive study of high-Tc interface superconductivity.” Journal of Physics and Chemistry of Solids (2009). In press. | J. A. Clayhold, O. Pelleg, A. T. Bollinger, G. Logvenov, B. M. Kerns, M. D. Schroer, D. W. Rench, and I. Bozovic. “Statistical Characterization and Process Control for Improved Growth of La2-xSrxCuO4 Films.” Journal of Superconductivity and Novel Magnetism 22, 797 (2009). Abstract We have used combinatorial molecular beam epitaxy (COMBE) technique to deposit thin cuprate
films with continuous spread in chemical composition, as well as nominally uniform films.
We have patterned them into linear pixel arrays and measured the transport properties of
each pixel. We applied detailed statistical analysis to differentiate between various possible
sources of random pixel-to-pixel variations, and utilized this knowledge to considerably tighten
the process parameters and significantly reduce such variations. The density and quality of data
points is high enough to allow detection of quantum phase transitions induced by tuning the chemical composition. | G. Logvenov, A. Gozar, and I. Bozovic. “High-Temperature Superconductivity in a Single Copper-Oxygen Plane.” Science 326, 699 (2009). Abstract The question of how thin cuprate layers can be while still retaining high-temperature
superconductivity (HTS) has been challenging to address, in part because experimental studies
require the synthesis of near-perfect ultrathin HTS layers and ways to profile the superconducting
properties such as the critical temperature and the superfluid density across interfaces with atomic
resolution. We used atomic-layer molecular beam epitaxy to synthesize bilayers of a cuprate metal
(La1.65Sr0.45CuO4) and a cuprate insulator (La2CuO4) in which each layer is just three unit cells
thick. We selectively doped layers with isovalent Zn atoms, which suppress superconductivity and
act as markers, to show that this interface HTS occurs within a single CuO2 plane. This approach
may also be useful in fabricating HTS devices. | I. Bozovic, G. Logvenov, A. Gozar, A. Bollinger, O. Pelleg, Z. Radovic, and N. Bozovic. “Nano-structured films of cuprate superconductors and other complex oxides: MBE synthesis, characterization, and engineered properties.” Proc. ICCE-17, ed. by D. Hui, 2009 (2009). Invited Keynote paper. | V. Y. Butko, G. Logvenov, N. Bozovic, Z. Radovic, and I. Bozovic. “Madelung Strain in Cuprate Superconductors - A Route to Enhancement of the Critical Temperature.” Advanced Materials 21, 3644 (2009). Abstract "Madelung Strain" in cuprate films containing metal (M = La1.56Sr0.44CuO4)
and insulator (I = La2CuO4) layers: X-ray diffraction shows that, unexpectedly, the volume
of unit cell of the top layer adjusts to that of the bottom layer. The effect is due to
long-range Coulomb forces; it affects interfacial superconductivity because the critical
temperature scales with the unit-cell height. | S. V. Dordevic, L. W. Kohlman, L. C. Tung, Y. J. Wang, A. Gozar, G. Logvenov, and I. Bozovic. “Absence of magnetic-field-induced effects in the mid-infrared transmission of La2-xSrxCuO4 thin films.” Physical Review B 79, 134503 (2009). Abstract We report magnetotransmission measurements on a series of La2-xSrxCuO4 thin films.
The measurements were performed in magnetic fields of 18 T, on films with doping levels
of x=0, 0.01, 0.03, 0.045, 0.06, 0.08, and 0.10. In addition, an optimally doped film
(x=0.16) was studied in magnetic fields up to 33 T, both above and below its superconducting
critical temperature Tc=41 K. A combination of Gaussian and wavelet filtering was
employed to improve the signal-to-noise ratio of the data. However, even after this
procedure, we could not detect any field-induced changes of transmission in any of
the studied samples. Our results therefore rule out a direct relation between intensity
changes in mid-infrared charge excitations and a bosonic mode in the far infrared.
We discuss these observations in the context of existing proposals regarding the
nature of medium energy range excitations in the cuprates. | S. Smadici, J. C. T. Lee, S. Wang, P. Abbamonte, G. Logvenov, A. Gozar, C. D. Cavellin, and I. Bozovic. “Superconducting Transition at 38 K in Insulating-Overdoped La2CuO4-La1.64Sr0.36CuO4 Superlattices: Evidence for Interface Electronic Redistribution from Resonant Soft X-Ray Scattering.” Physical Review Letters 102, 107004 (2009). Abstract We use resonant soft x-ray scattering (RSXS) to quantify the hole distribution in a superlattice of
insulating La2CuO4 (LCO) and overdoped La2-xSrxCuO4 (LSCO). Despite its nonsuperconducting constituents,
this structure is superconducting with Tc=38 K. We found that the conducting holes redistribute
electronically from LSCO to the LCO layers. The LCO layers were found to be optimally doped, suggesting
they are the main drivers of superconductivity. Our results demonstrate the utility of RSXS for
separating electronic from structural effects at oxide interfaces. | I. Bozovic, A. Gozar, G. Logvenov, A. Bollinger, N. Bozovic, and Z. Radovic. “Insights in High-Temperature Superconductivity from the Study of Films and Heterostructures Synthesized by Molecular Beam Epitaxy.” Journal of Superconductivity and Novel Magnetism 22, 223 (2009). Abstract Using molecular beam epitaxy, we synthesize atomically smooth thin films, multilayers and superlattices
of cuprate high-temperature superconductors (HTS). Such heterostructures enable novel experiments that probe
the basic physics of HTS. For example, we have established that HTS and antiferromagnetic phases separate
on Angstrom scale, while the pseudo-gap state apparently mixes with HTS over an anomalously large length scale
("Giant Proximity Effect"). Here, we briefly review our most recent experiments on such films
and superlattices. The new results include an unambiguous demonstration of strong coupling of in-plane
charge excitations to out-of-plane lattice vibrations and the discovery of interface HTS. |
2008
H. Shim, P. Chaudhari, G. Logvenov, and I. Bozovic. “Electron-Phonon Interactions in Superconducting La1.84Sr0.16CuO4 Films.” Physical Review Letters 101, 247004 (2008). Abstract We have measured quasiparticle tunneling across a junction perpendicular to the superconducting
copper oxide planes. The tunneling spectra show peaks in the density of states. There are 11 minima
in the second derivative d(2)I/dV(2), where I is the current and V the voltage, suggesting multiple
boson-quasiparticle interactions. These minima match precisely with the published Raman scattering
data, leading us to conclude that the relevant bosons in superconducting La1.84Sr0.16CuO4 films are
phonons. | N. Bozovic, I. Bozovic, and J. Misewich. “X-ray Nanocrystallography of Individual Carbon Nanotubes.” Nano Letters 8, 4477 (2008). Abstract By numerical simulations, we show that with the next-generation synchrotron sources one
should be able to record within minutes an X-ray diffraction pattern of a single nanotube and
from it decipher its detailed atomic structure. In calculated diffractograms we can even
discern signatures of presence of a single adatom and locate its position. New synchrotrons
will allow the existing method of electron nanocrystallography to be undertaken using X-ray
beams, thus facilitating in situ environmental studies. | A. Gozar, G. Logvenov, L. F. Kourkoutis, A. T. Bollinger, L. A. Giannuzzi, D. A. Muller, and I. Bozovic. “High-temperature interface superconductivity between metallic and insulating copper oxides.” Nature 455, 782 (2008). Abstract The realization of high- transition- temperature (high-Tc) superconductivity confined
to nanometre-sized interfaces has been a long- standing goal because of potential
applications(1,2) and the opportunity to study quantum phenomena in reduced dimensions(3,4).
This has been, however, a challenging target: in conventional metals, the high electron
density restricts interface effects (such as carrier depletion or accumulation) to a
region much narrower than the coherence length, which is the scale necessary for superconductivity
to occur. By contrast, in copper oxides the carrier density is low whereas Tc is high and the
coherence length very short, which provides an opportunity - but at a price: the interface must
be atomically perfect. Here we report superconductivity in bilayers consisting of an insulator
(La2CuO4) and a metal (La1.55Sr0.45CuO4), neither of which is superconducting in isolation.
In these bilayers, Tc is either similar to 15 K or similar to 30 K, depending on the layering
sequence. This highly robust phenomenon is confined within 2 - 3nm of the interface. If such a
bilayer is exposed to ozone, Tc exceeds 50 K, and this enhanced superconductivity is also shown
to originate from an interface layer about 1 - 2 unit cells thick. Enhancement of Tc in bilayer
systems was observed previously(5) but the essential role of the interface was not recognized at
the time. | I. Bozovic. “Experiments with atomically smooth thin films of cuprate superconductors: strong electron - phonon coupling and other surprises.” Physics-Uspekhi 51, 170 (2008). Proceedings Paper. Abstract This paper is based on a presentation prepared for the
Scientific Session of the Physical Sciences Department of the
Russian Academy of Sciences held on October 4, 2007, in
honor of 90th birthday of Academician V L Ginzburg. A
short review is presented of our own work only, including
some very recent experiments, on molecular beam epitaxy of
thin films of high-temperature superconductors (HTS). We
have developed a technique to fabricateHTS heterostructures
with atomically smooth surfaces and interfaces. This has
enabled a series of novel or improved experiments that
allowed us to address several key questions related to the
physics of HTS such as how it relates to the proximal
antiferromagnetic and pseudo-gap states. Firm experimental
evidence is obtained that in HTS cuprates coupling of
electrons to certain lattice vibrations is very strong. Next, we
have observed interface superconductivity in bilayers consist-
ing of two non-superconducting cuprates; the superfluid is
confined within a 1 – 2 unit-cell-thick layer next to the
interface. In the long run, this discovery may revive the
search for HTS in novel metal-insulator (MIMIM...) super-
lattices in which the mobile electrons in metallic layers are
paired by interaction that is enhanced in or originates from
the dielectric layers, as it was proposed by VLG forty years
ago. | G. Logvenov, V. V. Butko, C. DevilleCavellin, J. Seo, A. Gozar, and I. Bozovic. “Engineering interfaces in cuprate superconductors.” Physica B-Condensed Matter 403, 1149 (2008). Proceedings Paper. Abstract Using an advanced molecular beam epitaxy system for atomic-layer engineering of complex oxides, we have
fabricated a variety of superlattices with stacked layers of La2-xSrxCuO4 doped to different levels. In
superlattices formed by stacking highly overdoped, metallic La1.5Sr0.5CuO4 and insulating La2CuO4 layers
we have observed superconductivity at temperature as high as 30 K, even though neither of the building
blocks was superconducting. Different possible mechanisms of this superconductivity are discussed.
(C) 2007 Elsevier B.V. All rights reserved. | J. A. Clayhold, B. M. Kerns, M. D. Schroer, D. W. Rench, G. Logvenov, A. T. Bollinger, and I. Bozovic. “Combinatorial measurements of Hall effect and resistivity in oxide films.” Review of Scientific Instruments 79, 033908 (2008). Abstract A system for the simultaneous measurement of the Hall effect in 31 different locations as well
as the measurement of the resistivity in 30 different locations on a single oxide thin film grown
with a composition gradient is described. Considerations for designing and operating a high-throughput
system for characterizing highly conductive oxides with Hall coefficients as small as 10(-10) m(3)/C
are discussed. Results from measurements on films grown using combinatorial molecular beam epitaxy
show the usefulness of characterizing combinatorial libraries via both the resistivity and the Hall
effect. (C) 2008 American Institute of Physics. | Z. Radovic, N. Bozovic, and I. Bozovic. “Photoinduced expansion of cuprate superconductors: Evidence of strong electron-lattice coupling.” Physical Review B 77, 092508 (2008). Abstract Intense light pulses cause "colossal" (larger than thermal) expansion in cuprate films.
We show that a simple ionic model accounts for both the static expansion caused by chemical doping and the
photoinduced dynamic expansion. Because of the (static or intermittent) reduction of ionic charges, the
ions feel lesser electrostatic attraction and move apart. The displacements can be large since the Madelung
energy can change by more than 1 eV/atom. From this ionic picture, one would expect significant structural
reconstruction at heterointerfaces and photoinduced shifts in Raman frequencies. | G. Logvenov, and I. Bozovic. “Artificial superlattices grown by MBE: Could we design novel superconductors?” Physica C-Superconductivity and its Applications 468, 100 (2008). Abstract We review briefly the technique of atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) that allows
us to deposit atomically smooth single-crystal thin films of cuprate superconductors and various other
complex oxides. We give a couple of examples showing how this technique has been used to synthesize
'artificial' high-Tc superconductors - metastable high-n members of known cuprate families or
superlattices of various known (natural, stable) compounds. We also report briefly on the present
status of our state-of-the-art ALL-MBE system, and suggest that this technique could be useful in
search for new natural or artificial superconducting materials. (c) 2007 Elsevier B.V. All rights reserved. |
2007
G. Logvenov, I. Sveklo, and I. Bozovic. “Combinatorial molecular beam epitaxy of La2-xSrxCuO4+δ” Physica C-Superconductivity and its Applications 460, 416 (2007). Proceedings Paper. Abstract Using combinatorial molecular beam epitaxy (COMBE), we have deposited combinatorial libraries
of La2-xSrxCuO4 (LSCO) single-crystal thin films with different Sr content. A study of these LSCO
libraries allows for a systematic evaluation of the COMBE method. We have also developed and
tested a multiple-probe set-up for transport measurements that allowed us to measure within one
week the R(T) dependence in more than 2000 different samples (pixels in the ID combinatorial
library of LSCO). We also studied in detail the dependence of the crystal structure
(specifically, the c-axis lattice constant) on the Sr content and on the type of epitaxial
strain (compressive versus tensile). For the films grown on LaSrAlO4 substrates, we found
that the c-axis lattice constant of LSCO films decreased as Sr content was increased. This
we attribute to the reduction in the epitaxial strain, since the in-plane lattice constant
of LSCO decreases as Sr doping is increased. We also discuss the effect of the (small)
nonlinearity of beam profile. Published by Elsevier B.V. | J. Demsar, A. Gozar, V. K. Thorsmolle, A. J. Taylor, and I. Bozovic. “Long-lived near-infrared photoinduced absorption in LaSrAlO4 excited with visible light.” Physical Review B 76, 054304 (2007). Abstract Here, we report on studies of photoinduced carrier dynamics in LaSrAlO4. In samples annealed
in a reductive atmosphere, intense illumination by sub-band-gap photons results in strong photoinduced
absorption (PIA) centered at about 0.7 eV. As a function of temperature, PIA disappears above 200 K,
but at 4 K, it has a lifetime of several hours. The nonlinear dependence of PIA on light fluence
enables "writing" or "erasing" this optical memory effect by using visible and
infrared photons, respectively. Our results can be explained within a three-level scheme and are
discussed in a scenario that involves trapping of photoexcited electrons at oxygen-vacancy sites. | A. Gozar, G. Logvenov, V. Y. Butko, and I. Bozovic. “Surface structure analysis of atomically smooth BaBiO3 Films.” Physical Review B 75, 201402 (2007). Abstract Using low-energy time-of-flight scattering and recoil spectroscopy (TOFSARS) and mass spectroscopy
of recoiled ions (MSRI) we analyze the surface structure of an atomically smooth BaBiO3 film grown by
molecular beam epitaxy. We demonstrate high sensitivity of the TOFSARS and MSRI spectra to slight changes
in the orientation of the ion scattering plane with respect to the crystallographic axes. The observed
angle dependence allows us to clearly identify the termination layer as BiO2. Our data also indicate
that angle-resolved MSRI data can be used for high-resolution studies of the surface structure of
complex oxide thin films. | J. Q. He, R. F. Klie, G. Logvenov, I. Bozovic, and Y. M. Zhu. “Microstructure and possible strain relaxation mechanisms of La2CuO4+δ thin films grown on LaSrAlO4 and SrTiO3 substrates.” Journal of Applied Physics 101, 073906 (2007). Abstract Layered perovskite La2CuO4+δ (LCO) thin films were epitaxially grown on SrTiO3 (STO) and LaSrAlO4
substrates by atomic-layer-by-layer molecular beam epitaxy. The lattice defects and residual strain in these
films were investigated by means of transmission electron microscopy and electron energy loss spectroscopy.
The LCO films showed a high epitaxial quality with flat interfaces and top surfaces. Misfit dislocations
with Burgers vector a < 010 > and shear defects were frequently observed at or near the film/substrate
interfaces and in the films, respectively. In one LCO film, grown on STO at the highest temperature, 700
degrees C, we observed a two layered structure with the top layer being rhombohedral CuLaO2. In addition,
stacking faults were observed in the plane views of one LCO film grown on the STO substrate. The residual
strains were evaluated from the associated splitting of Bragg reflection. Possible mechanisms of strain
relaxations are discussed based on the observed defects. (c) 2007 American Institute of Physics. | I. Bozovic. “Possible Jahn-Teller effect and strong electron-phonon coupling in beryllium hydride.” High Tc Superconductors and Related Transition Metal Oxides, A. Bussmann-Holder, H. Keller (Eds.), Springer-Verlag Berlin, 43 (2007). | N. Gedik, D. S. Yang, G. Logvenov, I. Bozovic, and A. H. Zewail. “Nonequilibrium phase transitions in cuprates observed by ultrafast electron crystallography.” Science 316, 425 (2007). Abstract Nonequilibrium phase transitions, which are defined by the formation of macroscopic transient
domains, are optically dark and cannot be observed through conventional temperature- or pressure-change
studies. We have directly determined the structural dynamics of such a nonequilibrium phase transition
in a cuprate superconductor. Ultrafast electron crystallography with the use of a tilted optical geometry
technique afforded the necessary atomic-scale spatial and temporal resolutions. The observed transient behavior
displays a notable "structural isosbestic" point and a threshold effect for the dependence of
c-axis expansion (Delta c) on fluence (F), with Delta c/F = 0.02 angstrom/(millijoule per square
centimeter). This threshold for photon doping occurs at similar to 0.12 photons per copper site,
which is unexpectedly close to the density (per site) of chemically doped carriers needed to induce
superconductivity. |
2006
I. Bozovic. “About physics, myself, and Ginzburgs.” Journal of Superconductivity and Novel Magnetism 19, 469 (2006). Editorial Material. | G. Logvenov, and I. Bozovic. “Molecular Beam Epitaxy of Complex Oxides.” Proc. 2nd International conference On problems of High Temperature Superconductivity, I. Mitsin (Ed.), Zvenigorod, Russia (2006). |
2005
I. Bozovic, and D. Pavuna (editors). “Strongly Correlated Electron Materials: Physics and Nanoengineering.” SPIE, Bellingham (2005). | G. Logvenov, I. Sveklo, and I. Bozovic. “Combinatorial molecular beam epitaxy of La2-xSrxCuO4” SPIE Proceedings 5932 (2005). |
2004
I. Bozovic, G. Logvenov, M. A. J. Verhoeven, P. Caputo, E. Goldobin, and M. R. Beasley. “Giant proximity effect in cuprate superconductors.” Physical Review Letters 93, 157002 (2004). Abstract Using an advanced molecular beam epitaxy system, we have reproducibly synthesized atomically smooth films of high-temperature superconductors and uniform trilayer junctions with virtually perfect interfaces. We found that supercurrent runs through very thick barriers. We can rule out pinholes and microshorts; this "giant proximity effect" (GPE) is intrinsic. It defies the conventional explanation; it might originate in resonant tunneling through pair states in an almost-superconducting barrier. GPE may also be significant for superconducting electronics, since thick barriers are easier to fabricate. |
Last Modified: December 24, 2009
Please forward all questions about this site to:
Robert Sundling
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