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R. T. Gordon

Publications and source records attributed to R. T. Gordon.

At least 19 recordsLinked to original sources
arXiv2022

Quasiparticle tunneling as a probe of Josephson junction barrier and capacitor material in superconducting qubits

Non-equilibrium quasiparticles are possible sources for decoherence in superconducting qubits because they can lead to energy decay or dephasing upon tunneling across Josephson junctions (JJs). Here, we investigate the impact of the intrinsic properties of two-dimensional transmon qubits on quasiparticle tunneling (QPT) and discuss how we can use quasiparticle dynamics to gain critical information about the quality of JJ barrier. We find the tunneling rate of the nonequilibrium quasiparticles to be sensitive to the choice of the shunting capacitor material and their geometry in qubits. In some devices, we observe an anomalous temperature dependence of the QPT rate below 100 mK that deviates from a constant background associated with non-equilibrium quasiparticles. We speculate that this behavior is caused by high transmission sites/defects within the oxide barriers of the JJs, leading to spatially localized subgap states. We model this by assuming that such defects generate regions with a smaller effective gap. Our results present a unique in situ characterization tool to assess the uniformity of tunnel barriers in qubit junctions and shed light on how quasiparticles can interact with various elements of the qubit circuit.

quant-ph↗
arXiv2021

Environmental Radiation Impact on Lifetimes and Quasiparticle Tunneling Rates of Fixed-Frequency Transmon Qubits

Quantum computing relies on the operation of qubits in an environment as free of noise as possible. This work reports on measuring the impact of environmental radiation on lifetimes of fixed frequency transmon qubits with various capacitor pad geometries by varying the amount of shielding used in the measurement space. It was found that the qubit lifetimes are robust against these shielding changes until the most extreme limit was tested without a mixing chamber shield in the refrigerator. In contrast, the quasiparticle tunneling rates were found to be extremely sensitive to all configurations tested, indicating these devices are not yet limited by losses related to superconducting quasiparticles.

quant-ph↗
arXiv2013

Conventional superconductivity and hysteretic Campbell penetration depth in single crystals MgCNi$_3$

Single crystals of MgCNi$_3$, with areas sized up to 1 mm$^{2}$, were grown by the self flux method using a cubic anvil high pressure technique. The first critical field \textit{H$_{c1}$}, determined from a zero temperature extrapolation, is around 18 mT. Using the tunnel - diode resonator technique, the London penetration depth was measured with no applied \textit{dc} field and the Campbell penetration depth was measured with the external \textit{dc} fields up to 9T for two different sample orientations with respect to the direction of applied magnetic field. The absolute value of the London penetration depth, $λ(0) = 245 \pm 10$ nm was determined from the thermodynamic Rutgers formula. The superfluid density, $ρ_s=(λ(0)/λ(T))^2$ was found to follow the clean isotropic \textit{s}-wave behavior predicted by the weak - coupling BCS theory in the whole temperature range. The low - temperature behavior of the London penetration depth fits the BCS analytic form as well and produces close to the weak - coupling value of $Δ(0)/k_BT_c = 1.71$. The temperature dependence of the upper critical field, \textit{$H_{c2}$}, was found to be isotropic with a slope at \textit{T$_c$} of -2.63 T/K and \textit{H$_{c2}$}(0) $\approx$ 12.3 T at zero temperature. The Campbell penetration depth probes the vortex lattice response in the mixed state and is sensitive to the details of the pinning potential. For MgCNi$_3$, an irreversible feature has been observed in the TDR response when the sample is field-cooled and warmed versus zero-field-cooled and warmed. This feature possesses a non-monotonic field dependence and has commonly been referred to as the peak effect and is most likely related to a field - dependent non - parabolic pinning potential.

cond-mat.supr-con↗
arXiv2012

From d-wave to s-wave pairing in the iron-pnictide superconductor (Ba,K)Fe2As2

The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is s-wave and the Tc is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and pairing mechanism. We also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes.

cond-mat.supr-con↗
arXiv2012

Universal heat conduction in the iron-arsenide superconductor KFe2As2 : Evidence of a d-wave state

The thermal conductivity of the iron-arsenide superconductor KFe2As2 was measured down to 50 mK for a heat current parallel and perpendicular to the tetragonal c-axis. A residual linear term (RLT) at T=0 is observed for both current directions, confirming the presence of nodes in the superconducting gap. Our value of the RLT in the plane is equal to that reported by Dong et al. [Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity was ten times larger. This independence of the RLT on impurity scattering is the signature of universal heat transport, a property of superconducting states with symmetry-imposed line nodes. This argues against an s-wave state with accidental nodes. It favors instead a d-wave state, an assignment consistent with five additional properties: the magnitude of the critical scattering rate for suppressing Tc to zero; the magnitude of the RLT, and its dependence on current direction and on magnetic field; the temperature dependence of the thermal conductivity.

cond-mat.supr-con↗
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.

cond-mat.supr-con↗
arXiv2010

London penetration depth in Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$ (T=Co, Ni) superconductors irradiated with heavy ions

Irradiation with Pb ions was used to study the effect of disorder on the in-plane London penetration depth, $λ(T)$, in single crystals of Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$ (T=Co, Ni). An increase of the irradiation dose results in a monotonic decrease of the superconducting transition temperature, $T_c$, without affecting much the transition width. In both Co and Ni doped systems we find a power-law behavior, $Δλ(T) \propto T^n$, with the exponent $n$ systematically decreasing with the increase of disorder. This observation, supported by the theoretical analysis, conclusively points to a nodeless $s^\pm$ state with pairbreaking impurity scattering (interband) with strength being intermediate between Born and unitary limits.

cond-mat.supr-con↗
arXiv2010

London penetration depth and superfluid density in single crystals of Fe(Te,Se) and Fe(Te,S) superconductors

The in-plane London penetration depth, $λ(T)$, was measured in single crystals of the iron-chalcogenide superconductors Fe$_{1.03}$(Te$_{0.63}$Se$_{0.37}$) and Fe$_{1.06}$(Te$_{0.88}$S$_{0.14}$) by using a radio-frequency tunnel diode resonator. As is also the case for the iron-pnictides, these iron-chalcogenides exhibit a nearly quadratic temperature variation of $λ(T)$ at low temperatures. The absolute value of the penetration depth in the $T \to 0$ limit was determined for Fe$_{1.03}$(Te$_{0.63}$Se$_{0.37})$ by using an Al coating technique, giving $λ(0)\approx560 \pm 20$ nm. The superfluid density $ρ_s(T)=λ^2(0)/λ^2(T)$ was fitted with a self-consistent two-gap $γ-$model. While two different gaps are needed to describe the full-range temperature variation of $ρ_s(T)$, a non-exponential behavior at low temperatures requires additional factors, such as scattering and/or significant gap anisotropy.

cond-mat.supr-con↗
arXiv2009

London penetration depth and strong pair-breaking in iron-based superconductors

The low temperature variation of the London penetration depth for a number of iron-pnictide and iron-chalcogenide superconductors is nearly quadratic, $Δλ(T) = βT^n$ with $n\approx 2$. The coefficient in this dependence shows a robust scaling, $β\propto 1/T_c^3$ across different families of these materials. We associate the scaling with a strong pair-breaking. The same mechanism have recently been suggested to explain the scalings of the specific heat jump, $ΔC \propto T_c^3$, and of the slopes of the upper critical field, $dH_{c2}/dT\propto T_c$ in these materials. This suggests that thermodynamic and electromagnetic properties of the iron-based superconductors can be described within a strong pair-breaking scenario.

cond-mat.supr-con↗
arXiv2009

Three-dimensional nodal superconducting gap in single crystals Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$

The London penetration depth, $λ$, is directly related to the density, $n_{s}$, of the Cooper pairs ($λ^{2}\propto 1/n_{s}$) and its variation with temperature provides valuable insight into the pairing mechanism. Here we study the evolution with doping of the temperature dependence of the in-plane ($λ_{ab}$) and out-of-plane ($λ_{c}$) penetration depths in single crystals of electron-doped Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$. As is the case for other pnictides, $λ(T) \sim T^n$ over the whole doping range and this behavior extends down to at least $T=T_c/100$, setting a very small upper limit on the gap minimum. Furthermore, in the overdoped regime: 1) the exponent $n$ becomes substantially smaller than 2, which is incompatible with the models that explain power-law behavior to be due to scattering; 2) the exponent $n$ becomes anisotropic, with $λ_{c}(T)$ showing a clear $T$-linear behavior over a large temperature interval. These findings suggest that in the overdoped regime the superconducting gap in iron-based pnictide superconductors develops nodal structure, which unlike in the cuprates, cannot be understood within a two-dimensional picture.

cond-mat.supr-con↗
arXiv2009

Non-exponential London penetration depth in RFeAsO$_{0.9}$F$_{0.1}$ (R=La,Nd) single crystals

The superconducting penetration depth, $λ(T)$, has been measured in RFeAsO$_{0.9}$F$_{0.1}$ (R=La,Nd) single crystals (R-1111). In Nd-1111, we find an upturn in $λ(T)$ upon cooling and attribute it to the paramagnetism of the Nd ions, similar to the case of the electron-doped cuprate Nd-Ce-Cu-O. After the correction for paramagnetism, the London penetration depth variation is found to follow a power-law behavior, $Δλ_L(T)\propto T^{2}$ at low temperatures. The same $T^2$ variation of $λ(T)$ was found in non-magnetic La-1111 crystals. Analysis of the superfluid density and of penetration depth anisotropy over the full temperature range is consistent with two-gap superconductivity. Based on this and on our previous work, we conclude that both the RFeAsO (1111) and BaFe$_2$As$_2$ (122) families of pnictide superconductors exhibit unconventional two-gap superconductivity.

cond-mat.supr-con↗
arXiv2009

Non-exponential London penetration depth in Ba$_{1-}$K$_{x}$Fe$_{2}$As$_{2}$ single crystals

We have studied the in- and out-of-plane magnetic penetration depths in the hole- doped iron based superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ ($T_{c}\approx$ 30K). The study was performed on single crystals grown from different fluxes and we find that the results are nearly the same. The in-plane London penetration depth $λ_{ab}$ does not show exponential saturation at low temperature, as would be expected from a fully gapped superconductor. Instead, $λ_{ab}(T)$ shows a power-law behavior, $λ\propto T^{n}$ ($n\approx 2$), down to $T\approx 0.02 T_{c}$, similar to the electron doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. The penetration depth anisotropy $γ_λ=λ_{c}(T)/λ_{ab}(T)$ increases upon cooling, opposite to the trend observed in the anisotropy of the upper critical field, $γ_ξ=H_{c2}^{\perp c}(0)/H_{c2}^{\parallel c}(0)$. These are universal characteristics of both the electron and hole doped 122 systems, suggesting unconventional superconductivity. The behavior of the in-plane superfluid density $ρ_{ab}(T)$ is discussed in light of existing theoretical models proposed for the iron pnictides superconductors.

cond-mat.supr-con↗
arXiv2009

Doping dependent Irreversible Magnetic Properties of Ba(Fe1-xCox)2As2 Single Crystals

We discuss the irreversible magnetic properties of self-flux grown Ba(Fe1-xCox)2As2 single crystals for a wide range of concentrations covering the whole phase diagram from the underdoped to the overdoped regime, x=0.038, 0.047, 0.058, 0.071, 0.074, 0.10, 0.106 and 0.118. Samples were characterized by a magneto-optical method and show excellent spatial uniformity of the superconducting state. The overall behavior closely follows classical Bean model of the critical state. The field-dependent magnetization exhibits second peak at a temperature and doping - dependent magnetic field, Hp. The evolution of this fishtail feature with doping is discussed. Magnetic relaxation is time-logarithmic and unusually fast. Similar to cuprates, there is an apparent crossover from collective elastic to plastic flux creep above Hp. At high fields, the field dependence of the relaxation rate becomes doping independent. We discuss our results in the framework of the weak collective pinning and show that vortex physics in iron-based pnictide crystals is much closer to high-Tc cuprates than to conventional s-wave (including MgB2) superconductors.

cond-mat.supr-con↗
arXiv2009

Anisotropic London Penetration Depth and Superfluid Density in Single Crystals of Iron-based Pnictide Superconductors

In- and out-of-plane magnetic penetration depths were measured in three iron-based pnictide superconducting systems. All studied samples of both 122 systems show a robust power-law behavior, $λ(T) T^n$, with the sample-dependent exponent n=2-2.5, which is indicative of unconventional pairing. This scenario could be possible either through scattering in a $s_{\pm }$ state or due to nodes in the superconducting gap. In the Nd-1111 system, the interpretation of data may be obscured by the magnetism of rare-earth ions. The overall anisotropy of the pnictide superconductors is small. The 1111 system is about two times more anisotropic than the 122 system. Our data and analysis suggest that the iron-based pnictides are complex superconductors in which a multiband three-dimensional electronic structure and strong magnetic fluctuations play important roles.

cond-mat.supr-con↗
arXiv2008

Anisotropy of the Optimally-Doped Iron Pnictide Superconductor Ba(Fe0.926Co0.074)2As2

Anisotropies of electrical resistivity, upper critical field, London penetration depth and critical currents have been measured in single crystals of the optimally doped iron pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, $x$=0.074 and $T_c \sim$23 K. The normal state resistivity anisotropy was obtained by employing both the Montgomery technique and direct measurements on samples cut along principal crystallographic directions. The ratio $γ_ρ = ρ_c /ρ_a$ is about 4$\pm$1 just above $T_c$ and becomes half of that at room temperature. The anisotropy of the upper critical field, $γ_{H} = H_{c2,ab} /H_{c2,c} $, as determined from specific heat measurements close to $T_c$, is in the range of 2.1 to 2.6, depending on the criterion used. A comparable low anisotropy of the London penetration depth, $γ_λ=λ_{c}/λ_{ab}$, was recorded from TDR measurements and found to persist deep into the superconducting state. An anisotropy of comparable magnitude was also found in the critical currents, $γ_j=j_{c,ab}/j_{c,c}$, as determined from both direct transport measurements ($\sim$1.5) and from the analysis of the magnetization data ($\sim$3). Overall, our results show that iron pnictide superconductors manifest anisotropies consistent with essentially three-dimensional intermetallic compound and bear little resemblance to cuprates.

cond-mat.supr-con↗
arXiv2008

The London penetration depth in single crystals of Ba(Fe_{1-x}Co_x)_2As_2 at various doping levels

The London penetration depth $λ(T)$ has been measured in single crystals of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ using the tunnel diode resonator technique. The measured doping levels of $x=$~0.038, 0.047, 0.058, 0.074 and 0.10 range from underdoped to overdoped concentrations. The measurements have shown that the density of carriers participating in superconductivity decreases sharply in the underdoped regime, but the penetration depth as a function of temperature exhibits a robust power law, $Δλ(T)\sim T^{n}$, for all measured dopings, with $n$ between 2 and 2.5. We discuss the implications of these results and possible interpretations of such robust behavior.

cond-mat.supr-con↗
arXiv2008

Unconventional London penetration depth in Ba(Fe0.93Co0.07)2As2 single crystals

The London penetration depth, $λ(T)$, has been measured in several single crystals of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. Thermodynamic, electromagnetic, and structural characterization measurements confirm that these crystals are of excellent quality. The observed low temperature variation of $λ(T)$ follows a power-law, $Δλ(T) \sim T^n$ with $n=2.4 \pm 0.1$, indicating the existence of normal quasiparticles down to at least $0.02T_c$. This is in contrast to recent penetration depth measurements on single crystals of NdFeAsO$_{1-x}$F$_x$ and SmFeAsO$_{1-x}$F$_x$, which indicate an anisotropic but nodeless gap. We propose that a more three-dimensional character in the electronic structure of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ may lead to an anisotropic $s-$wave gap with point nodes that would explain the observed $λ(T)$.

cond-mat.supr-con↗