tine greibe | Technical University of Denmark (DTU) (original) (raw)

Papers by tine greibe

Research paper thumbnail of Are pinholes the cause of excess current in superconducting tunnel junctions? A study of Andreev current in highly resistive junctions

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed a... more In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic "pinholes" in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlOx/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlOx/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.

Research paper thumbnail of Improvement of chip design to reduce resonances in subgap regime of Joseph son junctions

ABSTRACT Excess current peaks in the IV curves of SIS Josephson junctions have been observed by s... more ABSTRACT Excess current peaks in the IV curves of SIS Josephson junctions have been observed by some groups [1-3]. These peaks have the shape of a resonance as a function of voltage. The resonances appear in the subgap regime of the junctions and the subgap current (leakage current) is concealed. The positions of the resonances do not change as a magnetic field is applied to the junctions, but their amplitude decreases when the supercurrent is suppressed. We have measured the subgap current of Al/AlOx/Al junctions and we show that these resonances are due to resonant modes in the chip design which are excited by the ac-Josephson effect. We present a chip design that decreases the amplitude of the resonances to a such degree that the subgap current is quantifiable.

Research paper thumbnail of Improvement of chip design to reduce resonances in subgap regime of Josephson junctions

Journal of Physics: Conference Series, 2009

Excess current peaks in the IV curves of SIS Josephson junctions have been observed by some group... more Excess current peaks in the IV curves of SIS Josephson junctions have been observed by some groups [1-3]. These peaks have the shape of a resonance as a function of voltage. The resonances appear in the subgap regime of the junctions and the subgap current (leakage current) is concealed. The positions of the resonances do not change as a magnetic field is applied to the junctions, but their amplitude decreases when the supercurrent is suppressed. We have measured the subgap current of Al/AlOx/Al junctions and we show that these resonances are due to resonant modes in the chip design which are excited by the ac-Josephson effect. We present a chip design that decreases the amplitude of the resonances to a such degree that the subgap current is quantifiable.

Research paper thumbnail of Are

In highly resistive superconductor---insulator---superconductor (SIS) and superconductor---insula... more In highly resistive superconductor---insulator---superconductor (SIS) and superconductor---insulator---normal-metal (SIN) junctions, "excess" subgap current is usually observed. We have studied subgap conductance in Al/AlOx/Al and Al/AlOx/Cu tunnel junctions. In the former, we observed a huge (two orders of magnitude) decrease in subgap conductance upon the transition from the SIS to the SIN regime. In the latter, we observed several signatures of coherent diffusive two-particle transport. We use the quasiclassical Keldysh-Green function theory to quantify the contributions of the single- and two-particle processes on subgap conductance. Our observations indicate insignificance of highly transparent microscopic defects ("pinholes") in the tunneling barrier, and we therefore argue that the common "pinhole" scenario is not the explanation for the observed excess subgap current in SIS tunnel junctions.

Research paper thumbnail of Are ``Pinholes'' the Cause of Excess Current in Superconducting Tunnel Junctions? A Study of Andreev Current in Highly Resistive Junctions

Physical Review Letters, 2011

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed a... more In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic pinholes in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlOx/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlOx/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement with experiment over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.

Research paper thumbnail of A Transition in the Pairing Symmetry of the Electron-Doped Cuprates

The pairing symmetry of the electron-doped cuprates is an important and controversial topic. We p... more The pairing symmetry of the electron-doped cuprates is an important and controversial topic. We present high precision, low frequency (50 kHz) measurements of the absolute value of the magnetic penetration depth, λ-2(T), from 1.6 K through Tc in numerous high-quality Pr_2-xCe_xCuO4 and La_2-xCe_xCuO4 films at a variety of doping levels, x. There are significant qualitative and quantitative changes in the behavior of the superfluid density, n_s(T) ∝ λ-2(T), near optimal doping. From study of the low-T temperature dependence of n_s(T), we associate these changes with a transition in pairing symmetry from d-wave at underdoping to s-wave at optimal and overdoping.

Research paper thumbnail of Role of substrates in molecular beam epitaxy growth of superconducting T$prime;La2$minus;xCexCuO4 films

Physica C-superconductivity and Its Applications, 2002

Research paper thumbnail of Phase control of La2CuO4 in thin film synthesis

Physical Review B, 2002

The lanthanum copper oxide, La2CuO4, which is an end member of the prototype high-Tc superconduct... more The lanthanum copper oxide, La2CuO4, which is an end member of the prototype high-Tc superconductors (La,Sr)2CuO4 and (La,Ba)2CuO4, crystallizes in the "K2NiF4" structure in high-temperature bulk synthesis. The crystal chemistry, however, predicts that La2CuO4 is at the borderline of the K2NiF4 stability and that it can crystallize in the Nd2CuO4 structure at low synthesis temperatures. In this article we demonstrate that low-temperature thin-film synthesis actually crystallizes La2CuO4 in the Nd2CuO4 structure. We also show that the phase control of "K2NiF4"-type La2CuO4 versus "Nd2CuO4"-type La2CuO4 can be achieved by varying the synthesis temperature and using different substrates.

Research paper thumbnail of Phase control in La214 epitaxial thin films

Research paper thumbnail of Pinning as a function of doping in e-doped cuprate superconductor La_2-xCe_xCuO_4

We have measured the vortex pinning strength in thin films of electron-doped cuprate La_2-xCe_xCu... more We have measured the vortex pinning strength in thin films of electron-doped cuprate La_2-xCe_xCuO4 for various doping x from x=0.075 (T_c=25K) to x=0.15, (T_c=12.5K). Optimal doping for this compound is x≈ 0.11, at which T_c=29K. We use a two-coil technique at frequencies from 10 to 100kHz. Zero temperature superfluid density n_s(0) grows rapidly with x on the underdoped side and decreases slowly on the overdoped side. The Labusch parameter, κ - the spring constant for a pinned vortex - shows large variations as doping changes. On the underdoped side it scales with n_s(0) to a maximum near optimal doping. With overdoping it decreases much more rapidly then n_s(0). Even at optimal doping κ (0) is two orders of magnitude lower then the highest values observed in YBCO.

Research paper thumbnail of Vortex pinning in electron-doped cuprate superconductor La2-xCexCuO4

Physica Status Solidi B-basic Solid State Physics, 2003

We have measured the vortex pinning strength in thin films of the electron-doped cuprate La2–xCex... more We have measured the vortex pinning strength in thin films of the electron-doped cuprate La2–xCexCuO4 for various doping x from x = 0.075 (Tc = 25 K) to x = 0.15 (Tc = 12.5 K). The optimal doping for this compound is ≈0.11, at which Tc = 29 K. Films were prepared by a molecular-beam epitaxy method. A structural XRD investigation revealed no doping-induced lattice strain. We use a two-coil technique at frequencies from 10 to 100 kHz. The superfluid density ns(0) grows rapidly with x on the underdoped side and decreases slowly on the overdoped side. The Labusch parameter κ, i.e. the spring constant for a pinned vortex, shows large variations as the doping changes. It increases with x rapidly to a maximum at slight underdoping and then it decreases on the overdoped side more rapidly than ns(0). Even at optimal doping κ (0) is an order of magnitude lower than the highest values observed in YBCO.

Research paper thumbnail of Magnetic Penetration Depth Measurements of MBE-Grown Pr_2-xCe_xCuO_4-delta Films on Pr_2CuO_4/SrTiO3 Substrates: Evidence of Nodeless Gap on the Fermi Surface

Inverse squared magnetic penetration depth, λ-2(T), in the basal plane of electron-doped Pr_2-xCe... more Inverse squared magnetic penetration depth, λ-2(T), in the basal plane of electron-doped Pr_2-xCe_xCuO_4-δ (0.115<= x <= 0.152) superconducting films was measured by using a two-coil mutual inductance technique at 50 kHz. The films were deposited on Pr_2CuO4 buffer layer/SrTiO3 (001) substrates via a MBE method. λ-2(0) of these films are about 25% higher than those of Pr_2-xCe_xCuO_4-δ films without the buffer layer, indicating even better quality of our films. Except for the lowest and highest doping (x=0.115 and 0.152), λ-2(T) at low temperatures shows a clear exponential temperature dependence. Our experimental results suggest a nodeless gap on the Fermi surface over most of, if not all of, the doping range studied. We found that the minimum values of the superconducting gap for the films are in range of 0.29<=Δ_min/k_BT_c<=1.0.

Research paper thumbnail of Evidence for a Transition in the Pairing Symmetry of the Electron-Doped Cuprates La2-xCexCuO4-y and Pr2-xCexCuO4-y

Physical Review Letters, 2002

We present measurements of the magnetic penetration depth, \lambda^{-2}(T), in Pr_{2-x}Ce_{x}CuO_... more We present measurements of the magnetic penetration depth, \lambda^{-2}(T), in Pr_{2-x}Ce_{x}CuO_{4-y} and La_{2-x}Ce_{x}CuO_{4-y} films at three Ce doping levels, x, near optimal. Optimal and overdoped films are qualitatively and quantitatively different from underdoped films. For example, \lambda^{-2}(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, \lambda^{-2}(T) at low T is exponential at optimal and overdoping but is quadratic at underdoping. In light of other studies that suggest both d- and s-wave pairing symmetry in nominally optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.

Research paper thumbnail of Evidence for a Nodeless Gap from the Superfluid Density of Optimally Doped Pr1.855Ce0.145CuO4-y Films

Physical Review Letters, 2002

We present measurements of the ab-plane magnetic penetration depth, \lambda(T), in five optimally... more We present measurements of the ab-plane magnetic penetration depth, \lambda(T), in five optimally doped Pr_{1.855}Ce_{0.145}CuO_{4-y} films for 1.6 K \leq T \leq T_c \sim 24 K. Low resistivities, high superfluid densities n_s(T)\propto \lambda^{-2}(T), high T_c's, and small transition widths are reproducible and indicative of excellent film quality. For all five films, \lambda^{-2}(T)/\lambda^{-2}(0) at low T is well fitted by an exponential temperature dependence with a gap, \Delta_{min}, of 0.85 k_B T_c. This behavior is consistent with a nodeless gap and is incompatible with d-wave superconductivity.

Research paper thumbnail of Evidence for a Nodeless Gap from the Superfluid Density of Optimally Doped Pr1.855Ce0.145CuO4-y Films

Physical Review Letters, 2002

We present measurements of the ab-plane magnetic penetration depth, λ(T), in five optimally doped... more We present measurements of the ab-plane magnetic penetration depth, λ(T), in five optimally doped Pr1.855Ce0.145CuO4-y films for 1.6 K<=T<=Tc~24 K. Low resistivities, high superfluid densities ns(T)~λ-2(T), high Tc's, and small transition widths are reproducible and indicative of excellent film quality. For all five films, λ-2(T)/λ-2(0) at low T is well fitted by an exponential temperature dependence with a gap, Δmin, of 0.85kBTc. This behavior is consistent with a nodeless gap and is incompatible with d-wave superconductivity.

Research paper thumbnail of Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La_{2-x}Ce_xCuO_{4-y} and Pr_{2-x}Ce_xCuO_{4-y

From measurements of the magnetic penetration depth, lambda(T)\lambda(T)lambda(T), from 1.6 K to TcT_cTc in films o... more From measurements of the magnetic penetration depth, lambda(T)\lambda(T)lambda(T), from 1.6 K to TcT_cTc in films of electron-doped cuprates La$_{2-x}$Ce$_x$CuO$_{4-y}$ and Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ we obtain the normalized density of states, Ns(E)N_s(E)Ns(E) at T=0 by using a simple model. In this framework, the flat behavior of lambda−2(T)\lambda^{-2}(T)lambda−2(T) at low TTT implies Ns(E)N_s(E)Ns(E) is small, possibly gapped, at low energies. The upward curvature in lambda−2(T)\lambda^{-2}(T)lambda−2(T) near TcT_cTc seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3kBTc3k_BT_c3kBTc of the Fermi surface.

Research paper thumbnail of Role of substrates in molecular beam epitaxy growth of superconducting T ′La 2− x Ce x CuO 4 films

Physica C-superconductivity and Its Applications, 2002

Research paper thumbnail of Phase control in La214 epitaxial thin films

The lanthanide (Ln) copper oxides of the general chemical formula Ln2CuO4 take two different crys... more The lanthanide (Ln) copper oxides of the general chemical formula Ln2CuO4 take two different crystal structures: K2NiF4(T) and Nd2CuO4(T'). La2CuO4 takes the T structure by high-temperature bulk processes. The "thermal expansion mismatch" between the La-O and Cu-O bonds predicts that the T' phase of La2CuO4 can be stabilized at synthesis temperatures below 425 deg. Such low synthesis temperatures are difficult to access by bulk processes, but easy by thin-film processes. We have surveyed growth parameters in molecular beam epitaxy, and succeeded in the selective stabilization of T- and T'-La2CuO4. From our observations, it turns out that the growth temperature as well as the substrate play a crucial role in the selective stabilization: the T' structure is stabilized at low growth temperatures (< 600 deg.) and with substrates of a < 3.70 Ang or a > 3.90 Ang, while the T structure is stabilized at high growth temperatures (> 650 deg) or with substrates of a ~ 3.70 - 3.85 Ang. We have also been attempting hole (Ca, Sr, and Ba) and electron (Ce) doping into both of T- and T'-La2CuO4. In T-La2CuO4, hole doping produces the well-known LSCO and LBCO. Surprisingly, contrary to the empirical law, electron doping is also possible up to x ~ 0.06 - 0.08, although the films do not show superconductivity. In T'-La2CuO4, electron doping produces superconducting T'-(La,Ce)2CuO4 with Tc ~ 30 K, although hole doping has as yet been unsuccessful.

Research paper thumbnail of Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La2 xCexCuO4 y and Pr2 xCexCuO4 y

Research paper thumbnail of Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La_{2-x}Ce_xCuO_{4-y} and Pr_{2-x}Ce_xCuO_{4-y

From measurements of the magnetic penetration depth, lambda(T)\lambda(T)lambda(T), from 1.6 K to TcT_cTc in films o... more From measurements of the magnetic penetration depth, lambda(T)\lambda(T)lambda(T), from 1.6 K to TcT_cTc in films of electron-doped cuprates La$_{2-x}$Ce$_x$CuO$_{4-y}$ and Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ we obtain the normalized density of states, Ns(E)N_s(E)Ns(E) at T=0 by using a simple model. In this framework, the flat behavior of lambda−2(T)\lambda^{-2}(T)lambda−2(T) at low TTT implies Ns(E)N_s(E)Ns(E) is small, possibly gapped, at low energies. The upward curvature in lambda−2(T)\lambda^{-2}(T)lambda−2(T) near TcT_cTc seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3kBTc3k_BT_c3kBTc of the Fermi surface.

Research paper thumbnail of Are pinholes the cause of excess current in superconducting tunnel junctions? A study of Andreev current in highly resistive junctions

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed a... more In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic "pinholes" in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlOx/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlOx/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.

Research paper thumbnail of Improvement of chip design to reduce resonances in subgap regime of Joseph son junctions

ABSTRACT Excess current peaks in the IV curves of SIS Josephson junctions have been observed by s... more ABSTRACT Excess current peaks in the IV curves of SIS Josephson junctions have been observed by some groups [1-3]. These peaks have the shape of a resonance as a function of voltage. The resonances appear in the subgap regime of the junctions and the subgap current (leakage current) is concealed. The positions of the resonances do not change as a magnetic field is applied to the junctions, but their amplitude decreases when the supercurrent is suppressed. We have measured the subgap current of Al/AlOx/Al junctions and we show that these resonances are due to resonant modes in the chip design which are excited by the ac-Josephson effect. We present a chip design that decreases the amplitude of the resonances to a such degree that the subgap current is quantifiable.

Research paper thumbnail of Improvement of chip design to reduce resonances in subgap regime of Josephson junctions

Journal of Physics: Conference Series, 2009

Excess current peaks in the IV curves of SIS Josephson junctions have been observed by some group... more Excess current peaks in the IV curves of SIS Josephson junctions have been observed by some groups [1-3]. These peaks have the shape of a resonance as a function of voltage. The resonances appear in the subgap regime of the junctions and the subgap current (leakage current) is concealed. The positions of the resonances do not change as a magnetic field is applied to the junctions, but their amplitude decreases when the supercurrent is suppressed. We have measured the subgap current of Al/AlOx/Al junctions and we show that these resonances are due to resonant modes in the chip design which are excited by the ac-Josephson effect. We present a chip design that decreases the amplitude of the resonances to a such degree that the subgap current is quantifiable.

Research paper thumbnail of Are

In highly resistive superconductor---insulator---superconductor (SIS) and superconductor---insula... more In highly resistive superconductor---insulator---superconductor (SIS) and superconductor---insulator---normal-metal (SIN) junctions, "excess" subgap current is usually observed. We have studied subgap conductance in Al/AlOx/Al and Al/AlOx/Cu tunnel junctions. In the former, we observed a huge (two orders of magnitude) decrease in subgap conductance upon the transition from the SIS to the SIN regime. In the latter, we observed several signatures of coherent diffusive two-particle transport. We use the quasiclassical Keldysh-Green function theory to quantify the contributions of the single- and two-particle processes on subgap conductance. Our observations indicate insignificance of highly transparent microscopic defects ("pinholes") in the tunneling barrier, and we therefore argue that the common "pinhole" scenario is not the explanation for the observed excess subgap current in SIS tunnel junctions.

Research paper thumbnail of Are ``Pinholes'' the Cause of Excess Current in Superconducting Tunnel Junctions? A Study of Andreev Current in Highly Resistive Junctions

Physical Review Letters, 2011

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed a... more In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic pinholes in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlOx/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlOx/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement with experiment over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.

Research paper thumbnail of A Transition in the Pairing Symmetry of the Electron-Doped Cuprates

The pairing symmetry of the electron-doped cuprates is an important and controversial topic. We p... more The pairing symmetry of the electron-doped cuprates is an important and controversial topic. We present high precision, low frequency (50 kHz) measurements of the absolute value of the magnetic penetration depth, λ-2(T), from 1.6 K through Tc in numerous high-quality Pr_2-xCe_xCuO4 and La_2-xCe_xCuO4 films at a variety of doping levels, x. There are significant qualitative and quantitative changes in the behavior of the superfluid density, n_s(T) ∝ λ-2(T), near optimal doping. From study of the low-T temperature dependence of n_s(T), we associate these changes with a transition in pairing symmetry from d-wave at underdoping to s-wave at optimal and overdoping.

Research paper thumbnail of Role of substrates in molecular beam epitaxy growth of superconducting T$prime;La2$minus;xCexCuO4 films

Physica C-superconductivity and Its Applications, 2002

Research paper thumbnail of Phase control of La2CuO4 in thin film synthesis

Physical Review B, 2002

The lanthanum copper oxide, La2CuO4, which is an end member of the prototype high-Tc superconduct... more The lanthanum copper oxide, La2CuO4, which is an end member of the prototype high-Tc superconductors (La,Sr)2CuO4 and (La,Ba)2CuO4, crystallizes in the "K2NiF4" structure in high-temperature bulk synthesis. The crystal chemistry, however, predicts that La2CuO4 is at the borderline of the K2NiF4 stability and that it can crystallize in the Nd2CuO4 structure at low synthesis temperatures. In this article we demonstrate that low-temperature thin-film synthesis actually crystallizes La2CuO4 in the Nd2CuO4 structure. We also show that the phase control of "K2NiF4"-type La2CuO4 versus "Nd2CuO4"-type La2CuO4 can be achieved by varying the synthesis temperature and using different substrates.

Research paper thumbnail of Phase control in La214 epitaxial thin films

Research paper thumbnail of Pinning as a function of doping in e-doped cuprate superconductor La_2-xCe_xCuO_4

We have measured the vortex pinning strength in thin films of electron-doped cuprate La_2-xCe_xCu... more We have measured the vortex pinning strength in thin films of electron-doped cuprate La_2-xCe_xCuO4 for various doping x from x=0.075 (T_c=25K) to x=0.15, (T_c=12.5K). Optimal doping for this compound is x≈ 0.11, at which T_c=29K. We use a two-coil technique at frequencies from 10 to 100kHz. Zero temperature superfluid density n_s(0) grows rapidly with x on the underdoped side and decreases slowly on the overdoped side. The Labusch parameter, κ - the spring constant for a pinned vortex - shows large variations as doping changes. On the underdoped side it scales with n_s(0) to a maximum near optimal doping. With overdoping it decreases much more rapidly then n_s(0). Even at optimal doping κ (0) is two orders of magnitude lower then the highest values observed in YBCO.

Research paper thumbnail of Vortex pinning in electron-doped cuprate superconductor La2-xCexCuO4

Physica Status Solidi B-basic Solid State Physics, 2003

We have measured the vortex pinning strength in thin films of the electron-doped cuprate La2–xCex... more We have measured the vortex pinning strength in thin films of the electron-doped cuprate La2–xCexCuO4 for various doping x from x = 0.075 (Tc = 25 K) to x = 0.15 (Tc = 12.5 K). The optimal doping for this compound is ≈0.11, at which Tc = 29 K. Films were prepared by a molecular-beam epitaxy method. A structural XRD investigation revealed no doping-induced lattice strain. We use a two-coil technique at frequencies from 10 to 100 kHz. The superfluid density ns(0) grows rapidly with x on the underdoped side and decreases slowly on the overdoped side. The Labusch parameter κ, i.e. the spring constant for a pinned vortex, shows large variations as the doping changes. It increases with x rapidly to a maximum at slight underdoping and then it decreases on the overdoped side more rapidly than ns(0). Even at optimal doping κ (0) is an order of magnitude lower than the highest values observed in YBCO.

Research paper thumbnail of Magnetic Penetration Depth Measurements of MBE-Grown Pr_2-xCe_xCuO_4-delta Films on Pr_2CuO_4/SrTiO3 Substrates: Evidence of Nodeless Gap on the Fermi Surface

Inverse squared magnetic penetration depth, λ-2(T), in the basal plane of electron-doped Pr_2-xCe... more Inverse squared magnetic penetration depth, λ-2(T), in the basal plane of electron-doped Pr_2-xCe_xCuO_4-δ (0.115<= x <= 0.152) superconducting films was measured by using a two-coil mutual inductance technique at 50 kHz. The films were deposited on Pr_2CuO4 buffer layer/SrTiO3 (001) substrates via a MBE method. λ-2(0) of these films are about 25% higher than those of Pr_2-xCe_xCuO_4-δ films without the buffer layer, indicating even better quality of our films. Except for the lowest and highest doping (x=0.115 and 0.152), λ-2(T) at low temperatures shows a clear exponential temperature dependence. Our experimental results suggest a nodeless gap on the Fermi surface over most of, if not all of, the doping range studied. We found that the minimum values of the superconducting gap for the films are in range of 0.29<=Δ_min/k_BT_c<=1.0.

Research paper thumbnail of Evidence for a Transition in the Pairing Symmetry of the Electron-Doped Cuprates La2-xCexCuO4-y and Pr2-xCexCuO4-y

Physical Review Letters, 2002

We present measurements of the magnetic penetration depth, \lambda^{-2}(T), in Pr_{2-x}Ce_{x}CuO_... more We present measurements of the magnetic penetration depth, \lambda^{-2}(T), in Pr_{2-x}Ce_{x}CuO_{4-y} and La_{2-x}Ce_{x}CuO_{4-y} films at three Ce doping levels, x, near optimal. Optimal and overdoped films are qualitatively and quantitatively different from underdoped films. For example, \lambda^{-2}(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, \lambda^{-2}(T) at low T is exponential at optimal and overdoping but is quadratic at underdoping. In light of other studies that suggest both d- and s-wave pairing symmetry in nominally optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.

Research paper thumbnail of Evidence for a Nodeless Gap from the Superfluid Density of Optimally Doped Pr1.855Ce0.145CuO4-y Films

Physical Review Letters, 2002

We present measurements of the ab-plane magnetic penetration depth, \lambda(T), in five optimally... more We present measurements of the ab-plane magnetic penetration depth, \lambda(T), in five optimally doped Pr_{1.855}Ce_{0.145}CuO_{4-y} films for 1.6 K \leq T \leq T_c \sim 24 K. Low resistivities, high superfluid densities n_s(T)\propto \lambda^{-2}(T), high T_c's, and small transition widths are reproducible and indicative of excellent film quality. For all five films, \lambda^{-2}(T)/\lambda^{-2}(0) at low T is well fitted by an exponential temperature dependence with a gap, \Delta_{min}, of 0.85 k_B T_c. This behavior is consistent with a nodeless gap and is incompatible with d-wave superconductivity.

Research paper thumbnail of Evidence for a Nodeless Gap from the Superfluid Density of Optimally Doped Pr1.855Ce0.145CuO4-y Films

Physical Review Letters, 2002

We present measurements of the ab-plane magnetic penetration depth, λ(T), in five optimally doped... more We present measurements of the ab-plane magnetic penetration depth, λ(T), in five optimally doped Pr1.855Ce0.145CuO4-y films for 1.6 K<=T<=Tc~24 K. Low resistivities, high superfluid densities ns(T)~λ-2(T), high Tc's, and small transition widths are reproducible and indicative of excellent film quality. For all five films, λ-2(T)/λ-2(0) at low T is well fitted by an exponential temperature dependence with a gap, Δmin, of 0.85kBTc. This behavior is consistent with a nodeless gap and is incompatible with d-wave superconductivity.

Research paper thumbnail of Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La_{2-x}Ce_xCuO_{4-y} and Pr_{2-x}Ce_xCuO_{4-y

From measurements of the magnetic penetration depth, lambda(T)\lambda(T)lambda(T), from 1.6 K to TcT_cTc in films o... more From measurements of the magnetic penetration depth, lambda(T)\lambda(T)lambda(T), from 1.6 K to TcT_cTc in films of electron-doped cuprates La$_{2-x}$Ce$_x$CuO$_{4-y}$ and Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ we obtain the normalized density of states, Ns(E)N_s(E)Ns(E) at T=0 by using a simple model. In this framework, the flat behavior of lambda−2(T)\lambda^{-2}(T)lambda−2(T) at low TTT implies Ns(E)N_s(E)Ns(E) is small, possibly gapped, at low energies. The upward curvature in lambda−2(T)\lambda^{-2}(T)lambda−2(T) near TcT_cTc seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3kBTc3k_BT_c3kBTc of the Fermi surface.

Research paper thumbnail of Role of substrates in molecular beam epitaxy growth of superconducting T ′La 2− x Ce x CuO 4 films

Physica C-superconductivity and Its Applications, 2002

Research paper thumbnail of Phase control in La214 epitaxial thin films

The lanthanide (Ln) copper oxides of the general chemical formula Ln2CuO4 take two different crys... more The lanthanide (Ln) copper oxides of the general chemical formula Ln2CuO4 take two different crystal structures: K2NiF4(T) and Nd2CuO4(T'). La2CuO4 takes the T structure by high-temperature bulk processes. The "thermal expansion mismatch" between the La-O and Cu-O bonds predicts that the T' phase of La2CuO4 can be stabilized at synthesis temperatures below 425 deg. Such low synthesis temperatures are difficult to access by bulk processes, but easy by thin-film processes. We have surveyed growth parameters in molecular beam epitaxy, and succeeded in the selective stabilization of T- and T'-La2CuO4. From our observations, it turns out that the growth temperature as well as the substrate play a crucial role in the selective stabilization: the T' structure is stabilized at low growth temperatures (< 600 deg.) and with substrates of a < 3.70 Ang or a > 3.90 Ang, while the T structure is stabilized at high growth temperatures (> 650 deg) or with substrates of a ~ 3.70 - 3.85 Ang. We have also been attempting hole (Ca, Sr, and Ba) and electron (Ce) doping into both of T- and T'-La2CuO4. In T-La2CuO4, hole doping produces the well-known LSCO and LBCO. Surprisingly, contrary to the empirical law, electron doping is also possible up to x ~ 0.06 - 0.08, although the films do not show superconductivity. In T'-La2CuO4, electron doping produces superconducting T'-(La,Ce)2CuO4 with Tc ~ 30 K, although hole doping has as yet been unsuccessful.

Research paper thumbnail of Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La2 xCexCuO4 y and Pr2 xCexCuO4 y

Research paper thumbnail of Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La_{2-x}Ce_xCuO_{4-y} and Pr_{2-x}Ce_xCuO_{4-y

From measurements of the magnetic penetration depth, lambda(T)\lambda(T)lambda(T), from 1.6 K to TcT_cTc in films o... more From measurements of the magnetic penetration depth, lambda(T)\lambda(T)lambda(T), from 1.6 K to TcT_cTc in films of electron-doped cuprates La$_{2-x}$Ce$_x$CuO$_{4-y}$ and Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ we obtain the normalized density of states, Ns(E)N_s(E)Ns(E) at T=0 by using a simple model. In this framework, the flat behavior of lambda−2(T)\lambda^{-2}(T)lambda−2(T) at low TTT implies Ns(E)N_s(E)Ns(E) is small, possibly gapped, at low energies. The upward curvature in lambda−2(T)\lambda^{-2}(T)lambda−2(T) near TcT_cTc seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3kBTc3k_BT_c3kBTc of the Fermi surface.