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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 T
2, respectively, the new defect scattering results suggest strongly that the two processes act as parallel conductance channels.
KeywordsElectrical transport-Resistivity-Cuprate superconductor-High temperature superconductor-LSCO-Normal state-Ion damage
Journal of Superconductivity and Novel Magnetism 04/2012; 23(3):339-342. · 0.65 Impact Factor
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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.
Journal of Superconductivity and Novel Magnetism 04/2012; 22(8):797-804. · 0.65 Impact Factor
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ABSTRACT: X-ray absorption in $\rm Sr_{2}CuO_{4-\delta}-La_{2}CuO_4$ (SCO-LCO)
superlattices shows a variable occupation with doping of a hole state different
from holes doped for $x \lesssim x_{optimal}$ in bulk $\rm La_{2-x}Sr_{x}CuO_4$
and suggests that this hole state is on apical oxygen atoms and polarized in
the $a-b$ plane. Considering the surface reflectivity gives a good qualitative
description of the line shapes of resonant soft X-ray scattering. The
interference between superlattice and surface reflections was used to
distinguish between scatterers in the SCO and the LCO layers, with the two hole
states maximized in different layers of the superlattice.
11/2011;
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ABSTRACT: Superlattices of the repeated structure La(1.56)Sr(0.44)CuO(4)/La(2)CuO(4)
(LSCO-LCO), where none of the constituents is superconducting, show a
superconducting transition of T_c \simeq 25 K. In order to elucidate the nature
of the superconducting state we have performed a low-energy muSR study. By
applying a magnetic field parallel (Meissner state) and perpendicular (vortex
state) to the film planes, we could show that superconductivity is sheet like,
resulting in a very anisotropic superconducting state. This result is
consistent with a simple charge-transfer model, which takes into account the
layered structure and the difference in the chemical potential between LCO and
LSCO, as well as Sr interdiffusion. Using a pancake-vortex model we could
estimate a strict upper limit of the London penetration depth to 380 nm in
these superlattices. The temperature dependence of the muon depolarization rate
in field cooling experiments is very similar to what is observed in
intercalated BSCCO and suggests that vortex-vortex interaction is dominated by
electromagnetic coupling but negligible Josephson interaction.
10/2011;
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ABSTRACT: We show, by means of low-energy muon spin rotation measurements, that
few-unit-cells thick La(2)CuO(4) layers synthesized digitally by molecular beam
epitaxy synthesis are antiferromagnetically ordered. Below a thickness of about
5 CuO(2) layers the long-range ordered state breaks down, and a magnetic state
appears with enhanced quantum fluctuations and a reduced spin stiffness. This
magnetic state can exist in close proximity (few Angstrom) to high-temperature
superconducting layers, without transmitting supercurrents.
10/2011;
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ABSTRACT: The nature of the underdoped pseudogap regime of the high-temperature
superconductors has been a matter of long-term debate. On quite general
grounds, one expects that due to their low superfluid densities and short
correlation lengths, superconducting fluctuations will be significant for
transport and thermodynamic properties in this part of the phase diagram.
Although there is ample experimental evidence for such correlations, there has
been disagreement about how high in temperature they may persist, their role in
the phenomenology of the pseudogap, and their significance for understanding
high-temperature superconductivity. In this work we use THz time-domain
spectroscopy (TTDS) to probe the temporal fluctuations of superconductivity
above the critical temperature (T$_c$) in La$_{2-x}$Sr$_x$CuO$_4$ thin films
over a doping range that spans almost the entire superconducting dome (x = 0.09
to 0.25). Signatures of the fluctuations persist in the conductivity in a
comparatively narrow temperature range, at most 16 K above Tc. Our measurements
show that superconducting correlations do not make an appreciable contribution
to the charge transport anomalies of the pseudogap in LSCO at temperatures well
above T$_c$.
10/2011;
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ABSTRACT: X-ray absorption spectroscopy and resonant soft x-ray reflectivity show a
non-uniform distribution of oxygen holes in a La2NiO4 - La2CuO4 (LNO-LCO)
superlattice, with excess holes concentrated in the LNO layers. Weak
ferromagnetism with Tc = 160 K suggests a coordinated tilting of NiO6
octahedra, similar to that of bulk LNO. Ni d3z2-r2 orbitals within the LNO
layers have a spatially variable occupation. This variation of the Ni valence
near LNO-LCO interfaces is observed with resonant soft x-ray reflectivity at
the Ni L edge, at a reflection suppressed by the symmetry of the structure, and
is possible through graded doping with holes, due to oxygen interstitials taken
up preferentially by inner LNO layers. Since the density of oxygen atoms in the
structure can be smoothly varied with standard procedures, this orbital
occupation, robust up to at least 280 K, is tunable.
06/2011;
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ABSTRACT: We present measurements of the fluctuation superconductivity in an underdoped
thin film of La$_{1.905}$Sr$_{0.095}$CuO$_4$ using time-domain THz
spectroscopy. We compare our results with measurements of diamagnetism in a
similarly doped crystal of La$_{2-x}$Sr$_x$CuO$_4$. We show through a
vortex-plasma model that if the fluctuation diamagnetism solely originates in
vortices, then they must necessarily exhibit an anomalously large vortex
diffusion constant, which is more than two orders of magnitude larger than the
Bardeen-Stephen estimate. This points to either the extremely unusual
properties of vortices in the under-doped d-wave cuprates or a contribution to
the diamagnetic response that is not superconducting in origin.
03/2011;
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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 (La(1.65)Sr(0.45)CuO4) 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.
Science 10/2009; 326(5953):699-702. · 31.20 Impact Factor
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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 T_{c}=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.
Physical Review Letters 04/2009; 102(10):107004. · 7.37 Impact Factor
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ABSTRACT: The realization of high-transition-temperature (high-T(c)) superconductivity confined to nanometre-sized interfaces has been a long-standing goal because of potential applications and the opportunity to study quantum phenomena in reduced dimensions. 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 T(c) 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 (La(2)CuO(4)) and a metal (La(1.55)Sr(0.45)CuO(4)), neither of which is superconducting in isolation. In these bilayers, T(c) is either approximately 15 K or approximately 30 K, depending on the layering sequence. This highly robust phenomenon is confined within 2-3 nm of the interface. If such a bilayer is exposed to ozone, T(c) exceeds 50 K, and this enhanced superconductivity is also shown to originate from an interface layer about 1-2 unit cells thick. Enhancement of T(c) in bilayer systems was observed previously but the essential role of the interface was not recognized at the time.
Nature 11/2008; 455(7214):782-5. · 36.28 Impact Factor
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ABSTRACT: High-temperature superconductivity confined to nanometer-size 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}. However, this is a challenging target: in conventional metals the high electron density restricts interface effects such as carrier depletion/accumulation to a region much narrower than the coherence length, the scale necessary for superconductivity to occur. In contrast, in copper oxides the carrier density is low while the critical temperature (T_c) is high and the coherence length very short; so, this provides a breakthrough opportunity but at a price: the interface must be atomically perfect. Here we report on superconductivity in bilayers consisting of an insulator (La_2CuO_4) and a metal (La_{1.55}Sr_{0.45}CuO_{4}), neither of which is superconducting in isolation. However, in bilayers T_c is either ~15 K or ~30 K, depending on the layering sequence. This highly robust phenomenon is confined within 2-3 nm from the interface. If such a bilayer is exposed to ozone, T_c exceeds 50 K and this enhanced superconductivity is also shown to originate from the interface layer about 1-2 unit cell thick. Enhancement of T_c in bilayer systems was observed previously^5 but the essential role of the interface was not recognized at the time. Our results demonstrate that engineering artificial heterostructures provides a novel, unconventional way to fabricate stable, quasi two-dimensional high T_c phases and to significantly enhance superconducting properties in known or new superconductors. Comment: authors' version. the published version can be found on Nature web site
10/2008;
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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) m3/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.
Review of Scientific Instruments 04/2008; 79(3):033908. · 1.37 Impact Factor
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ABSTRACT: Using low energy Time-of-Flight Scattering and Recoil Spectroscopy (TOF-SARS) and Mass Spectroscopy of Recoiled Ions (MSRI) we analyze the surface structure of an atomically smooth BaBiO$_3$ film grown by molecular beam epitaxy. We demonstrate high sensitivity of the TOF-SARS 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 BiO$_2$. Our data also indicate that angle-resolved MSRI data can be used for high resolution studies of surface structure of complex oxide thin films. Comment: 5 pages, 4 figures accepted for publication in Physical Review B
04/2007;
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ABSTRACT: An unresolved issue concerning cuprate superconductors is whether the distribution of carriers in the CuO2 plane is uniform or inhomogeneous. Because the carriers comprise a small fraction of the total charge density and may be rapidly fluctuating, modulations are difficult to detect directly. We demonstrate that in anomalous x-ray scattering at the oxygen K edge of the cuprates, the contribution of carriers to the scattering amplitude is selectively magnified 82 times. This enhances diffraction from the doped holes by more than 10(3), permitting direct structural analysis of the superconducting ground state. Scattering from thin films of La2CuO4+delta (superconducting transition temperature = 39 K) at temperature = 50 +/- 5 kelvin on the reciprocal space intervals (0,0,0.21) --> (0,0,1.21) and (0,0,0.6) --> (0.3,0,0.6) shows a rounding of the carrier density near the substrate suggestive of a depletion zone or similar effect. The structure factor for off-specular scattering was less than 3 x 10(-7) electrons, suggesting an absence of in-plane hole ordering in this material.
Science 07/2002; 297(5581):581-4. · 31.20 Impact Factor
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ABSTRACT: X-ray absorption spectroscopy and resonant soft x-ray reflectivity show a nonuniform distribution of oxygen holes in a La2NiO4-La2CuO4 (LNO-LCO) superlattice, with excess holes concentrated in the LNO layers. Weak ferromagnetism with Tc=160 K suggests a coordinated tilting of NiO6 octahedra, similar to that of bulk LNO. Ni d3z2−r2 orbitals within the LNO layers have a spatially variable occupation. This variation of the Ni valence near LNO-LCO interfaces is observed with resonant soft x-ray reflectivity at the Ni and Cu L edges, at a reflection suppressed by the symmetry of the structure, and is possible through graded doping with holes, due to oxygen interstitials taken up preferentially by inner LNO layers. Since the density of oxygen atoms in the structure can be smoothly varied with standard procedures, this orbital occupation, robust up to at least 280 K, is tunable.
Phys. Rev. B. 84(15).
