arXiv subjects

R. W. Giannetta

Publications and source records attributed to R. W. Giannetta.

At least 19 recordsLinked to original sources

arXiv2013

Effect of Heavy-Ion Irradiation on Superconductivity in Single Crystals of Ba0.6K0.4Fe2As2 Pnictide Superconductor

The London penetration depth was measured in optimally doped Ba0.6K0.4Fe2As2 crystals, with and without columnar defects produced by 1.4 GeV 208Pb irradiation. The low temperature behavior of unirradiated samples was consistent with a fully gapped superconducting state with a minimum energy gap delta_min/(k_B T_C) = 1. Similar gap values were observed for irradiation levels corresponding to mean column-column separations of 32 nm and 22 nm. At very high irradiation levels (column-column separation of 10 nm) a T^2 power law was observed below Tc/3, most likely due to elevated scattering. Neither the location nor the sharpness of the superconducting transition was affected by irradiation. The data provides evidence for an s+/- pairing state.

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arXiv2013

Nodal superconductivity in isovalently substituted SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ pnictide superconductor at the optimal doping, $x=$0.35

Temperature-dependent London penetration depth, $λ(T)$, was measured in optimally - doped, $x=$0.35, as-grown ($T_c \approx$25 K, RRR=$ρ(300K)/ρ(T_c)$=4.5) and annealed ($T_c \approx$35 K, RRR=6.4) single crystals of SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ iron - based superconductor. Annealing increases the RRR and decreases the absolute value of the London penetration depth from $λ(0) = 300 \pm 10$ nm in as-grown sample to $λ(0) = 275 \pm 10$ nm. At low temperatures, $λ(T) \sim T$ indicating superconducting gap with line nodes. Analysis of the full - temperature range superfluid density is consistent with the line nodes, but differs from the simple single - gap $d-$wave. The observed behavior is very similar to that of BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, showing that isovalently substituted pnictides are inherently different from the charge - doped materials.

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arXiv2012

A Sharp Peak of the Zero-Temperature Penetration Depth at Optimal Composition in BaFe2(As1-xPx)2

In a superconductor, the ratio of the carrier density, $n$, to their effective mass, $m^*$, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, $λ_L$. In two dimensional systems, this ratio $n/m^*$ ($\sim 1/λ_L^2$) determines the effective Fermi temperature, $T_F$. We report a sharp peak in the $x$-dependence of $λ_L$ at zero temperature in clean samples of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ at the optimum composition $x = 0.30$, where the superconducting transition temperature $T_c$ reaches a maximum of 30\,K. This structure may arise from quantum fluctuations associated with a quantum critical point (QCP). The ratio of $T_c/T_F$ at $x = 0.30$ is enhanced, implying a possible crossover towards the Bose-Einstein condensate limit driven by quantum criticality.

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arXiv2010

Doping evolution of the absolute value of the London penetration depth and superfluid density in single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

The zero temperature value of the in-plane London penetration depth, $λ_{ab}(0)$, has been measured in single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ as a function of the Co concentration, $x$, across both the underdoped and overdoped superconducting regions of the phase diagram. For $x\gtrsim0.047$, $λ_{ab}(0)$ has been found to have values between 120 $\pm$ 50~nm and 300 $\pm$ 50~nm. A pronounced increase in $λ_{ab}(0)$, to a value as high as 950 $\pm$ 50~nm, has been observed for $x\lesssim0.047$, corresponding to the region of the phase diagram where the itinerant antiferromagnetic and superconducting phases coexist and compete. Direct determination of the doping-dependent $λ_{ab}(0)$ has allowed us to track the evolution of the temperature-dependent superfluid density, from which we infer the development of a pronounced superconducting gap anisotropy at the edges of the superconducting dome.

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arXiv2007

Penetration depth study of superconducting gap structure of 2\textit{H}-NbSe$_2$

We report measurements of the temperature dependence of both in-plane and out-of-plane penetration depths ($λ_a$ and $λ_c$ respectively) in 2\textit{H}-NbSe$_2$. Measurements were made with a radio-frequency tunnel diode oscillator circuit at temperatures down to 100 mK. Analysis of the anisotropic superfluid density shows that a reduced energy gap is located on one or more of the quasi-two-dimensional Nb Fermi surface sheets rather than on the Se sheet, in contrast to some previous reports. This result suggests that the gap structure is not simply related to the weak electron-phonon coupling on the Se sheet and is therefore important for microscopic models of anisotropic superconductivity in this compound.

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arXiv2004

Field-dependent diamagnetic transition in magnetic superconductor $Sm_{1.85} Ce_{0.15} Cu O_{4-y}$

The magnetic penetration depth of single crystal $\rm{Sm_{1.85}Ce_{0.15}CuO_{4-y}}$ was measured down to 0.4 K in dc fields up to 7 kOe. For insulating $\rm{Sm_2CuO_4}$, Sm$^{3+}$ spins order at the Néel temperature, $T_N = 6$ K, independent of the applied field. Superconducting $\rm{Sm_{1.85}Ce_{0.15}CuO_{4-y}}$ ($T_c \approx 23$ K) shows a sharp increase in diamagnetic screening below $T^{\ast}(H)$ which varied from 4.0 K ($H = 0$) to 0.5 K ($H =$ 7 kOe) for a field along the c-axis. If the field was aligned parallel to the conducting planes, $T^{\ast}$ remained unchanged. The unusual field dependence of $T^{\ast}$ indicates a spin freezing transition that dramatically increases the superfluid density.

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arXiv2003

Nodal Order Parameter in Electron-Doped Superconducting Films Pr(2-x)Ce(x)CuO(4-y) (x=0.13, 0.15 and 0.17)

The London penetration depth, lambda{ab}(T), is reported for thin films of the electron-doped superconductor Pr{2-x}Ce{x}CuO{4-y} at three doping levels (x = 0.13, 0.15 and 0.17). Measurements down to 0.35 K were carried out using a tunnel diode oscillator with excitation fields applied both perpendicular and parallel to the conducting planes. For all samples and both field orientations lambda{ab}(T) showed power law behavior implying a superconducting gap with nodes.

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arXiv2002

Campbell Penetration Depth of a Superconductor in the Critical State

The magnetic penetration depth $λ(T,H,j)$ was measured in the presence of a slowly relaxing supercurrent, $j$. In single crystal $\mathrm{Bi_2Sr_2CaCu_2O_8}$ below approximately 25 K, $λ(T,H,j)$ is strongly hysteretic. We propose that the irreversibility arises from a shift of the vortex position within its pinning well as $j$ changes. The Campbell length depends upon the ratio $j/j_{c}$ where $j_{c}$ is the critical current defined through the Labusch parameter. Similar effects were observed in other cuprates and in an organic superconductor.

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arXiv2001

Energy gap and proximity effect in $MgB_2$ superconducting wires

Measurements of the penetration depth $λ(T,H)$ in the presence of a DC magnetic field were performed in $MgB_2$ wires. In as-prepared wires $λ(T,H<130 Oe)$ shows a strong diamagnetic downturn below $\approx 10 K$. A DC magnetic field of $130 Oe$ completely suppressed the downturn. The data are consistent with proximity coupling to a surface $Mg$ layer left during synthesis. A theory for the proximity effect in the clean limit, together with an assumed distribution of the $Mg$ layer thickness, qualitatively explains the field and temperature dependence of the data. Removal of the $Mg$ by chemical etching results in an exponential temperature dependence for $λ(T)$ with an energy gap of $2 Δ(0)/T_c\approx 1.54$ ($Δ(0) \approx 2.61 meV$), in close agreement with recent measurements on commercial powders and single crystals. This minimum gap is only 44% of the BCS weak coupling value, implying substantial anisotropy.

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arXiv2000

Evidence for Surface Andreev Bound states in Cuprate Superconductors from Penetration Depth Measurements

Tunneling and theoretical studies have suggested that Andreev bound states form at certain surfaces of unconventional superconductors. Through studies of the temperature and field dependence of the in-plane magnetic penetration depth lambda_ab at low temperature, we have found strong evidence for the presence of these states in clean single crystal YBCO and BSCCO. Crystals cut to expose a [110] interface show a strong upturn in lambda_ab at around 7K, when the field is oriented so that the supercurrents flow around this surface. In YBCO this upturn is completely suppressed by a field of ~0.1 T.

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arXiv2000

Unusual temperature dependence of the London penetration depth in all-organic β``-(ET)_2SF_5CH_2CF_2SO_3 single crystals

The temperature dependence of the in-plane, λ_{\parallel}, and interplane, λ_{\perp}, London penetration depth was measured in the metal-free all-organic superconductor β-ET (see title) ($T_c \approx$ 5.2 K). λ_{\parallel} ~T^3 up to 0.5 Tc, a power law previously observed only in materials thought to be p-wave superconductors. λ_{\perp} is larger than the sample dimensions down to the lowest temperatures (0.35 K), implying an anisotropy of λ_{\perp}/λ_{\parallel} ~ 400-800.

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arXiv2000

Measurements of the absolute value of the penetration depth in high-$ T_c$ superconductors using a tunnel diode resonator

A method is presented to measure the absolute value of the London penetration depth, $λ$, from the frequency shift of a resonator. The technique involves coating a high-$T_c$ superconductor (HTSC) with film of low - Tc material of known thickness and penetration depth. The method is applied to measure London penetration depth in YBa2Cu3O{7-δ} (YBCO) Bi2Sr2CaCu2O{8+δ} (BSCCO) and Pr{1.85}Ce{0.15}CuO{4-δ}$ (PCCO). For YBCO and BSCCO, the values of $λ(0)$ are in agreement with the literature values. For PCCO $λ\approx 2790$ Å, reported for the first time.

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arXiv2000

Evidence for nodal quasiparticles in electron-doped cuprates from penetration depth measurements

The in-plane magnetic penetration depth, $λ(T)$, was measured down to 0.4 K in single crystals of electron-doped superconductors, Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-δ}$ (PCCO) and Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-δ}$ (NCCO). In PCCO, the superfluid density varies as $T^2$ from 0.025 up to roughly 0.3 $T/T_c$ suggestive of a d-wave state with impurities. In NCCO, $λ(T)$ shows a pronounced upturn for $T < 4$ K due to the paramagnetic contribution of Nd$^{3+}$ ions. Fits to an s-wave order parameter over the standard BCS range ($T/T_c$ = 0.32) limit any gap to less than $Δ_{min} (0)/T_c = 0.57$ in NCCO. For PCCO, the absence of paramagnetism permits a lower temperature fit and yields an upper limit of $Δ_{min} (0)/T_c = 0.2$.

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arXiv2000

Nonequilibrium Campbell length: probing the vortex pinning potential

The $AC$ magnetic penetration depth $λ(T,H,j)$ was measured in presence of a macroscopic $DC$ (Bean) supercurrent, $j$. In single crystal BSCCO below approximately 28 K, $λ(T,H,j)$ exhibits thermal hysteresis. The irreversibility arises from a shift of the vortex position within its pinning well as $j$ changes. It is demonstrated that below a new irreversibility temperature, the nonequilibrium Campbell length depends upon the ratio $j/j_c$. $λ(T,H,j)$ {\it increases} with $j/j_c$ as expected for a non-parabolic potential well whose curvature {\it decreases} with the displacement. Qualitatively similar results are observed in other high-$T_{c}$ and conventional superconductors.

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arXiv2000

Low temperature vortex phase diagram of Bi2Sr2CaCu2O8 : a magnetic penetration depth study

We report measurements of the magnetic penetration depth λ_m(T) in the presence of a DC magnetic field in optimally doped BSCCO-2212 single crystals. Warming, after magnetic field is applied to a zero-field cooled sample, results in a non-monotonic λ_m(T), which does not coincide with a curve obtained upon field cooling, thus exhibiting a hysteretic behaviour. We discuss the possible relation of our results to the vortex decoupling, unbinding, and dimensional crossover.

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arXiv2000

Meissner - London state in superconductors of rectangular cross-section in perpendicular magnetic field

The distribution of magnetic induction in Meissner state with finite London penetration depth is analyzed for platelet samples of rectangular cross-section in a perpendicular magnetic field. The exact 2D numerical solution of the London equation is extended analytically to the realistic 3D case. Data obtained on Nb cylinders and foils as well as single crystals of YBCO and BSCCO are in a good agreement with the model. The results are particularly relevant for magnetic susceptibility, rf and microwave resonator measurements of the magnetic penetration depth in high-Tc superconductors.

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arXiv1999

Magnetic penetration depth in electron-doped cuprates - evidence for gap nodes

The in-plane penetration depth λ(T) is measured in electron-doped single crystals Nd1.85Ce0.15CuO4-x (NCCO) and Pr1.85Ce0.15CuO4-x (PCCO) using a 11 MHz LC resonator. In NCCO, λ(T) exhibits a minimum at 3.8 K and a pronounced upturn down to 0.4 K due to the paramagnetic contribution of Nd3+ ions. The London penetration depth contribution is linear in T. The paramagnetic contribution is absent in PCCO, where λ(T)~T^2 at low temperatures. Our results indicate the presence of nodes in the superconducting gap, i.e., non s-wave symmetry of the order parameter in electron-doped cuprates.

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arXiv1999

Low Temperature Penetration depth of κ-(ET)_2Cu[N(CN)_2]Br and κ-(ET)_2Cu(NCS)_2

We present high precision measurements of the penetration depth λof single crystals of κ-(ET)_2Cu[N(CN)2]Br and κ-(ET)_2Cu(NCS)_2 at temperature down to 0.4 K. We find that, at low temperatures, the in-plane penetration depth varies as a fractional power law, λ\sim T^1.5. W hilst this may be taken as evidence for novel bose excitation processes, we show that the data are also consistent with a quasi-linear variation of the superfluid density, as is expected for a d-wave superconductor with impurities. Our data for the interplane penetration depth show similar features and give a direct measurement of the absolute value, λ(0)=100\pm 20 μm.

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