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Ivan Bozovic - One of the best experts on this subject based on the ideXlab platform.
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spontaneous breaking of rotational symmetry in Copper oxide superconductors
Nature, 2017Co-Authors: A. T. Bollinger, Ivan BozovicAbstract:The origin of high-temperature superconductivity in Copper Oxides and the nature of the 'normal' state above the critical temperature are widely debated. In underdoped Copper Oxides, this normal state hosts a pseudogap and other anomalous features; and in the overdoped materials, the standard Bardeen-Cooper-Schrieffer description fails, challenging the idea that the normal state is a simple Fermi liquid. To investigate these questions, we have studied the behaviour of single-crystal La2-xSrxCuO4 films through which an electrical current is being passed. Here we report that a spontaneous voltage develops across the sample, transverse (orthogonal) to the electrical current. The dependence of this voltage on probe current, temperature, in-plane device orientation and doping shows that this behaviour is intrinsic, substantial, robust and present over a broad range of temperature and doping. If the current direction is rotated in-plane by an angle ϕ, the transverse voltage oscillates as sin(2ϕ), breaking the four-fold rotational symmetry of the crystal. The amplitude of the oscillations is strongly peaked near the critical temperature for superconductivity and decreases with increasing doping. We find that these phenomena are manifestations of unexpected in-plane anisotropy in the electronic transport. The films are very thin and epitaxially constrained to be tetragonal (that is, with four-fold symmetry), so one expects a constant resistivity and zero transverse voltage, for every ϕ. The origin of this anisotropy is purely electronic-the so-called electronic nematicity. Unusually, the nematic director is not aligned with the crystal axes, unless a substantial orthorhombic distortion is imposed. The fact that this anisotropy occurs in a material that exhibits high-temperature superconductivity may not be a coincidence.
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dependence of the critical temperature in overdoped Copper Oxides on superfluid density
Nature, 2016Co-Authors: Ivan Bozovic, A. T. BollingerAbstract:The scaling law for the critical temperature and zero-temperature stiffness in an overdoped Copper oxide semiconductor does not conform to the standard Bardeen–Cooper–Schrieffer description. Ivan Božovic et al. present a comprehensive study of the key physical properties of the overdoped Copper oxide superconductor La2−xSrxCuO4. Their results run counter to the common assumption that strongly correlated fermion physics evolves smoothly into conventional Bardeen–Cooper–Schrieffer (BCS) behaviour in overdoped Copper oxide superconductors. Rather, in La2−xSrxCuO4 the scaling law for the critical superconducting temperature and zero-temperature phase stiffness does not conform to standard BCS physics. The authors speculate that the high critical temperature derives from local electron pairing and unusual kinematics. The physics of underdoped Copper oxide superconductors, including the pseudogap, spin and charge ordering and their relation to superconductivity1,2,3, is intensely debated. The overdoped Copper Oxides are perceived as simpler, with strongly correlated fermion physics evolving smoothly into the conventional Bardeen–Cooper–Schrieffer behaviour. Pioneering studies on a few overdoped samples4,5,6,7,8,9,10,11 indicated that the superfluid density was much lower than expected, but this was attributed to pair-breaking, disorder and phase separation. Here we report the way in which the magnetic penetration depth and the phase stiffness depend on temperature and doping by investigating the entire overdoped side of the La2−xSrxCuO4 phase diagram. We measured the absolute values of the magnetic penetration depth and the phase stiffness to an accuracy of one per cent in thousands of samples; the large statistics reveal clear trends and intrinsic properties. The films are homogeneous; variations in the critical superconducting temperature within a film are very small (less than one kelvin). At every level of doping the phase stiffness decreases linearly with temperature. The dependence of the zero-temperature phase stiffness on the critical superconducting temperature is generally linear, but with an offset; however, close to the origin this dependence becomes parabolic. This scaling law is incompatible with the standard Bardeen–Cooper–Schrieffer description.
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superconductor insulator transition in la2 xsrxcuo4 at the pair quantum resistance
Nature, 2011Co-Authors: A. T. Bollinger, Guy Dubuis, Joonah Yoon, D Pavuna, James A Misewich, Ivan BozovicAbstract:High-temperature superconductivity in Copper Oxides arises when a parent insulator compound is 'doped' by adding or removing valence electrons, usually by inserting atoms into the lattice structure. This would be better achieved by tuning the carrier density using the electric field effect, as it removes ambiguity about whether the electronic properties change because of alterations in the crystal structure or in the electronic structure. Such tuning is difficult to achieve because it requires perfect, ultrathin films and a huge local field. Bollinger et al. report the synthesis of one-cell-thick epitaxial films of La2xSrxCuO4, and the use of the films to make double-layer transistors. The transistors have very large fields, and by changing the surface carrier density, the critical temperature can be shifted by up to 30 K. The resistance varies as predicted for a two-dimensional superconductor–insulator transition. High-temperature superconductivity in Copper Oxides arises when a parent insulator compound is doped beyond some critical concentration; what exactly happens at this superconductor–insulator transition is a key open question1. The cleanest approach is to tune the carrier density using the electric field effect2,3,4,5,6,7; for example, it was learned in this way5 that weak electron localization transforms superconducting SrTiO3 into a Fermi-glass insulator. But in the Copper Oxides this has been a long-standing technical challenge3, because perfect ultrathin films and huge local fields (>109 V m−1) are needed. Recently, such fields have been obtained using electrolytes or ionic liquids in the electric double-layer transistor configuration8,9,10. Here we report synthesis of epitaxial films of La2− xSr x CuO4 that are one unit cell thick, and fabrication of double-layer transistors. Very large fields and induced changes in surface carrier density enable shifts in the critical temperature by up to 30 K. Hundreds of resistance versus temperature and carrier density curves were recorded and shown to collapse onto a single function, as predicted for a two-dimensional superconductor–insulator transition11,12,13,14. The observed critical resistance is precisely the quantum resistance for pairs, RQ = h/(2e)2 = 6.45 kΩ, suggestive of a phase transition driven by quantum phase fluctuations, and Cooper pair (de)localization.
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high temperature interface superconductivity between metallic and insulating Copper Oxides
Nature, 2008Co-Authors: A Gozar, D. A. Muller, A. T. Bollinger, Lucille A. Giannuzzi, Gennady Logvenov, Fitting L Kourkoutis, Ivan BozovicAbstract:High-temperature interface superconductivity, confined to nanometre-size interfaces between two non-superconducting materials, has been long sought-after. Theorists speculated about it for forty years, but it has been elusive because of extreme demands on interface perfection and materials parameters. Gozar et al. now report the discovery of interface superconductivity between two non-superconducting cuprates — an insulator (La2CuO4) and a metal (La1.55Sr0.45CuO4). The critical tempeature (Tc) in bilayers is about 15 K or 30 K, depending on the layering sequence. This highly robust phenomenon is confined to within 2 or 3 nanometres of the interface, and if the bilayer is exposed to ozone, Tc rises to over 50 K. The realization of high-transition-temperature (high-Tc) superconductivity confined to nanometre-sized interfaces has been a long-standing goal because of potential applications1,2 and the opportunity to study quantum phenomena in reduced dimensions3,4. 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 ∼15 K or ∼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, 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 previously5 but the essential role of the interface was not recognized at the time.
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High-temperature interface superconductivity between metallic and insulating Copper Oxides
Nature, 2008Co-Authors: A Gozar, D. A. Muller, A. T. Bollinger, L. Fitting Kourkoutis, Lucille A. Giannuzzi, Gennady Logvenov, Ivan BozovicAbstract: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.
N.l. Wang - One of the best experts on this subject based on the ideXlab platform.
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nearly isotropic superconductivity in ba k fe 2 as 2
Nature, 2009Co-Authors: J. L. Luo, G Chen, H Q Yuan, J Singleton, F F Balakirev, S A Baily, N.l. WangAbstract:Superconductivity has been observed in iron-arsenic (or iron pnictide) compounds with critical transition temperatures as high as 56 K, inviting comparison with the high-Tc Copper Oxides. The Copper Oxides have quasi-two-dimensional electronic properties, which led to speculation that extreme anisotropy (direction dependence) is a requirement for superconductivity at temperatures above 40 K. Yuan et al. report that this is not the case: the superconducting properties of (Ba,K)Fe2As2 are in fact quite isotropic. This behaviour is strikingly different from all previously known layered superconductors. It is shown that the superconducting properties of (Ba,K)Fe2As2 are quite isotropic. Such behaviour is strikingly different from all previously known layered superconductors, and indicates that reduced dimensionality in these compounds is not a prerequisite for 'high-temperature' superconductivity. Superconductivity was recently observed1,2,3,4,5,6,7 in iron-arsenic-based compounds with a superconducting transition temperature (Tc) as high as 56 K, naturally raising comparisons with the high-Tc Copper Oxides. The Copper Oxides have layered crystal structures with quasi-two-dimensional electronic properties, which led to speculation that reduced dimensionality (that is, extreme anisotropy) is a necessary prerequisite for superconductivity at temperatures above 40 K (refs 8, 9). Early work on the iron-arsenic compounds seemed to support this view7,10. Here we report measurements of the electrical resistivity in single crystals of (Ba,K)Fe2As2 in a magnetic field up to 60 T. We find that the superconducting properties are in fact quite isotropic, being rather independent of the direction of the applied magnetic fields at low temperature. Such behaviour is strikingly different from all previously known layered superconductors9,11, and indicates that reduced dimensionality in these compounds is not a prerequisite for ‘high-temperature’ superconductivity. We suggest that this situation arises because of the underlying electronic structure of the iron-arsenic compounds, which appears to be much more three dimensional than that of the Copper Oxides. Extrapolations of low-field single-crystal data incorrectly suggest a high anisotropy and a greatly exaggerated zero-temperature upper critical field.
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Magnetic order close to superconductivity in the iron-based layered LaO1-xFxFeAs systems
Nature, 2008Co-Authors: Clarina De La Cruz, J. L. Luo, G F Chen, J L Zarestky, H. A. Mook, Jiying Li, William Ratcliff, J.w. Lynn, Q Huang, N.l. WangAbstract:Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) Copper Oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) Copper Oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.
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magnetic order close to superconductivity in the iron based layered lao1 xfxfeas systems
Nature, 2008Co-Authors: J L Zarestky, H. A. Mook, William Ratcliff, J.w. Lynn, Q Huang, Clarina Dela Cruz, G Chen, Jianlin Luo, N.l. WangAbstract:Much like high-Tc Copper Oxides, superconductivity in the newly discovered the rare-earth iron-based oxide systems is derived from either electron or hole doping of their non-superconducting parent compounds. The parent LaOFeAs material is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. While optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping electrons to form superconductivity, there has been no direct evidence of SDW order. de la Cruz et al. report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below ∼150 K, followed by the development of long-range SDW-type antiferromagnetic order at ∼137 K with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and structural distortion in favour of superconductivity. Therefore, like high-Tc Copper Oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range ordered antiferromagnetic ground state. Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-Tc) Copper Oxides1,2, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile ‘electrons’ or ‘holes’ are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron3,4,5,6,7 or hole8 doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility3. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity9, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at ∼137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure9. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-Tc Copper Oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.
A. T. Bollinger - One of the best experts on this subject based on the ideXlab platform.
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spontaneous breaking of rotational symmetry in Copper oxide superconductors
Nature, 2017Co-Authors: A. T. Bollinger, Ivan BozovicAbstract:The origin of high-temperature superconductivity in Copper Oxides and the nature of the 'normal' state above the critical temperature are widely debated. In underdoped Copper Oxides, this normal state hosts a pseudogap and other anomalous features; and in the overdoped materials, the standard Bardeen-Cooper-Schrieffer description fails, challenging the idea that the normal state is a simple Fermi liquid. To investigate these questions, we have studied the behaviour of single-crystal La2-xSrxCuO4 films through which an electrical current is being passed. Here we report that a spontaneous voltage develops across the sample, transverse (orthogonal) to the electrical current. The dependence of this voltage on probe current, temperature, in-plane device orientation and doping shows that this behaviour is intrinsic, substantial, robust and present over a broad range of temperature and doping. If the current direction is rotated in-plane by an angle ϕ, the transverse voltage oscillates as sin(2ϕ), breaking the four-fold rotational symmetry of the crystal. The amplitude of the oscillations is strongly peaked near the critical temperature for superconductivity and decreases with increasing doping. We find that these phenomena are manifestations of unexpected in-plane anisotropy in the electronic transport. The films are very thin and epitaxially constrained to be tetragonal (that is, with four-fold symmetry), so one expects a constant resistivity and zero transverse voltage, for every ϕ. The origin of this anisotropy is purely electronic-the so-called electronic nematicity. Unusually, the nematic director is not aligned with the crystal axes, unless a substantial orthorhombic distortion is imposed. The fact that this anisotropy occurs in a material that exhibits high-temperature superconductivity may not be a coincidence.
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dependence of the critical temperature in overdoped Copper Oxides on superfluid density
Nature, 2016Co-Authors: Ivan Bozovic, A. T. BollingerAbstract:The scaling law for the critical temperature and zero-temperature stiffness in an overdoped Copper oxide semiconductor does not conform to the standard Bardeen–Cooper–Schrieffer description. Ivan Božovic et al. present a comprehensive study of the key physical properties of the overdoped Copper oxide superconductor La2−xSrxCuO4. Their results run counter to the common assumption that strongly correlated fermion physics evolves smoothly into conventional Bardeen–Cooper–Schrieffer (BCS) behaviour in overdoped Copper oxide superconductors. Rather, in La2−xSrxCuO4 the scaling law for the critical superconducting temperature and zero-temperature phase stiffness does not conform to standard BCS physics. The authors speculate that the high critical temperature derives from local electron pairing and unusual kinematics. The physics of underdoped Copper oxide superconductors, including the pseudogap, spin and charge ordering and their relation to superconductivity1,2,3, is intensely debated. The overdoped Copper Oxides are perceived as simpler, with strongly correlated fermion physics evolving smoothly into the conventional Bardeen–Cooper–Schrieffer behaviour. Pioneering studies on a few overdoped samples4,5,6,7,8,9,10,11 indicated that the superfluid density was much lower than expected, but this was attributed to pair-breaking, disorder and phase separation. Here we report the way in which the magnetic penetration depth and the phase stiffness depend on temperature and doping by investigating the entire overdoped side of the La2−xSrxCuO4 phase diagram. We measured the absolute values of the magnetic penetration depth and the phase stiffness to an accuracy of one per cent in thousands of samples; the large statistics reveal clear trends and intrinsic properties. The films are homogeneous; variations in the critical superconducting temperature within a film are very small (less than one kelvin). At every level of doping the phase stiffness decreases linearly with temperature. The dependence of the zero-temperature phase stiffness on the critical superconducting temperature is generally linear, but with an offset; however, close to the origin this dependence becomes parabolic. This scaling law is incompatible with the standard Bardeen–Cooper–Schrieffer description.
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superconductor insulator transition in la2 xsrxcuo4 at the pair quantum resistance
Nature, 2011Co-Authors: A. T. Bollinger, Guy Dubuis, Joonah Yoon, D Pavuna, James A Misewich, Ivan BozovicAbstract:High-temperature superconductivity in Copper Oxides arises when a parent insulator compound is 'doped' by adding or removing valence electrons, usually by inserting atoms into the lattice structure. This would be better achieved by tuning the carrier density using the electric field effect, as it removes ambiguity about whether the electronic properties change because of alterations in the crystal structure or in the electronic structure. Such tuning is difficult to achieve because it requires perfect, ultrathin films and a huge local field. Bollinger et al. report the synthesis of one-cell-thick epitaxial films of La2xSrxCuO4, and the use of the films to make double-layer transistors. The transistors have very large fields, and by changing the surface carrier density, the critical temperature can be shifted by up to 30 K. The resistance varies as predicted for a two-dimensional superconductor–insulator transition. High-temperature superconductivity in Copper Oxides arises when a parent insulator compound is doped beyond some critical concentration; what exactly happens at this superconductor–insulator transition is a key open question1. The cleanest approach is to tune the carrier density using the electric field effect2,3,4,5,6,7; for example, it was learned in this way5 that weak electron localization transforms superconducting SrTiO3 into a Fermi-glass insulator. But in the Copper Oxides this has been a long-standing technical challenge3, because perfect ultrathin films and huge local fields (>109 V m−1) are needed. Recently, such fields have been obtained using electrolytes or ionic liquids in the electric double-layer transistor configuration8,9,10. Here we report synthesis of epitaxial films of La2− xSr x CuO4 that are one unit cell thick, and fabrication of double-layer transistors. Very large fields and induced changes in surface carrier density enable shifts in the critical temperature by up to 30 K. Hundreds of resistance versus temperature and carrier density curves were recorded and shown to collapse onto a single function, as predicted for a two-dimensional superconductor–insulator transition11,12,13,14. The observed critical resistance is precisely the quantum resistance for pairs, RQ = h/(2e)2 = 6.45 kΩ, suggestive of a phase transition driven by quantum phase fluctuations, and Cooper pair (de)localization.
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high temperature interface superconductivity between metallic and insulating Copper Oxides
Nature, 2008Co-Authors: A Gozar, D. A. Muller, A. T. Bollinger, Lucille A. Giannuzzi, Gennady Logvenov, Fitting L Kourkoutis, Ivan BozovicAbstract:High-temperature interface superconductivity, confined to nanometre-size interfaces between two non-superconducting materials, has been long sought-after. Theorists speculated about it for forty years, but it has been elusive because of extreme demands on interface perfection and materials parameters. Gozar et al. now report the discovery of interface superconductivity between two non-superconducting cuprates — an insulator (La2CuO4) and a metal (La1.55Sr0.45CuO4). The critical tempeature (Tc) in bilayers is about 15 K or 30 K, depending on the layering sequence. This highly robust phenomenon is confined to within 2 or 3 nanometres of the interface, and if the bilayer is exposed to ozone, Tc rises to over 50 K. The realization of high-transition-temperature (high-Tc) superconductivity confined to nanometre-sized interfaces has been a long-standing goal because of potential applications1,2 and the opportunity to study quantum phenomena in reduced dimensions3,4. 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 ∼15 K or ∼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, 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 previously5 but the essential role of the interface was not recognized at the time.
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High-temperature interface superconductivity between metallic and insulating Copper Oxides
Nature, 2008Co-Authors: A Gozar, D. A. Muller, A. T. Bollinger, L. Fitting Kourkoutis, Lucille A. Giannuzzi, Gennady Logvenov, Ivan BozovicAbstract: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.
A S Alexandrov - One of the best experts on this subject based on the ideXlab platform.
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bipolaron anisotropic flat bands hall mobility edge and metal semiconductor duality of overdoped high tc Oxides
Physical Review B, 1996Co-Authors: A S AlexandrovAbstract:Hole bipolaron band structure with two flat anisotropic bands is derived for oxide superconductors. Strong anisotropy leads to one-dimensional localization in a random field which explains the metal-like value of the Hall effect and the semiconductorlike doping dependence of resistivity of overdoped Oxides. Doping dependence of ${T}_{c}$ and ${\ensuremath{\lambda}}_{H}(0)$ as well as the low-temperature dependence of resistivity, of the Hall effect, ${H}_{c2}(T)$ and robust features of angle-resolved photoemission spectroscopy of several high-${T}_{c}$ Copper Oxides are explained.
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bipolaron anisotropic flat bands hall mobility edge and metal semiconductor duality of overdoped high tc Oxides
Physical Review B, 1996Co-Authors: A S AlexandrovAbstract:Hole bipolaron band structure with two flat anisotropic bands is derived for oxide superconductors. Strong anisotropy leads to one-dimensional localization in a random field which explains the {ital metal}-{ital like} value of the Hall effect and the {ital semiconductorlike} doping dependence of resistivity of overdoped Oxides. Doping dependence of {ital T}{sub {ital c}} and {lambda}{sub {ital H}}(0) as well as the low-temperature dependence of resistivity, of the Hall effect, {ital H}{sub {ital c}2}({ital T}) and robust features of angle-resolved photoemission spectroscopy of several high-{ital T}{sub {ital c}} Copper Oxides are explained. {copyright} {ital 1996 The American Physical Society.}
Clarina Dela Cruz - One of the best experts on this subject based on the ideXlab platform.
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magnetic order close to superconductivity in the iron based layered lao1 xfxfeas systems
Nature, 2008Co-Authors: J L Zarestky, H. A. Mook, William Ratcliff, J.w. Lynn, Q Huang, Clarina Dela Cruz, G Chen, Jianlin Luo, N.l. WangAbstract:Much like high-Tc Copper Oxides, superconductivity in the newly discovered the rare-earth iron-based oxide systems is derived from either electron or hole doping of their non-superconducting parent compounds. The parent LaOFeAs material is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. While optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping electrons to form superconductivity, there has been no direct evidence of SDW order. de la Cruz et al. report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below ∼150 K, followed by the development of long-range SDW-type antiferromagnetic order at ∼137 K with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and structural distortion in favour of superconductivity. Therefore, like high-Tc Copper Oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range ordered antiferromagnetic ground state. Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-Tc) Copper Oxides1,2, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile ‘electrons’ or ‘holes’ are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron3,4,5,6,7 or hole8 doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility3. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity9, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at ∼137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure9. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-Tc Copper Oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.