arXiv subjects

C. Kurter

Publications and source records attributed to C. Kurter.

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

Suppressed crosstalk between two-junction superconducting qubits with mode-selective exchange coupling

Fixed-frequency qubits can suffer from always-on interactions that inhibit independent control. Here, we address this issue by experimentally demonstrating a superconducting architecture using qubits that comprise of two capacitively-shunted Josephson junctions connected in series. Historically known as tunable coupling qubits (TCQs), such two-junction qubits support two modes with distinct frequencies and spatial symmetries. By selectively coupling only one type of mode and using the other as our computational basis, we greatly suppress crosstalk between the data modes while permitting all-microwave two-qubit gates.

quant-ph↗

arXiv2021

Merged-Element Transmons: Design and Qubit Performance

We have demonstrated a novel type of superconducting transmon qubit in which a Josephson junction has been engineered to act as its own parallel shunt capacitor. This merged-element transmon (MET) potentially offers a smaller footprint and simpler fabrication than conventional transmons. Because it concentrates the electromagnetic energy inside the junction, it reduces relative electric field participation from other interfaces. By combining micrometer-scale Al/AlOx/Al junctions with long oxidations and novel processing, we have produced functional devices with $E_{J}$/$E_{C}$ in the low transmon regime ($E_{J}$/$E_{C}$ $\lesssim$30). Cryogenic I-V measurements show sharp dI/dV structure with low sub-gap conduction. Qubit spectroscopy of tunable versions show a small number of avoided level crossings, suggesting the presence of two-level systems (TLS). We have observed mean T1 times typically in the range of 10-90 microseconds, with some annealed devices exhibiting T1 > 100 microseconds over several hours. The results suggest that energy relaxation in conventional, small-junction transmons is not limited by junction loss.

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↗

arXiv2016

Robust Fabry-Perot interference in dual-gated Bi$_2$Se$_3$ devices

We study Fabry-Perot interference in hybrid devices, each consisting of a mesoscopic superconducting disk deposited on the surface of a three-dimensional topological insulator. Such structures are hypothesized to contain protected zero modes known as Majorana fermions bound to vortices. The interference manifests as periodic conductance oscillations of magnitude $\sim 0.1$ $e^2/h$. These oscillations show no strong dependence on bulk carrier density or sample thickness, suggesting that they result from phase coherent transport in surface states. However, the Fabry-Perot interference can be tuned by both top and back gates, implying strong electrostatic coupling between the top and bottom surfaces of topological insulator.

cond-mat.mes-hall↗

arXiv2015

Evidence for an anomalous current phase relation in topological insulator Josephson junctions

Josephson junctions with topological insulator weak links can host low energy Andreev bound states giving rise to a current phase relation that deviates from sinusoidal behaviour. Of particular interest are zero energy Majorana bound states that form at a phase difference of $π$. Here we report on interferometry studies of Josephson junctions and superconducting quantum interference devices (SQUIDs) incorporating topological insulator weak links. We find that the nodes in single junction diffraction patterns and SQUID oscillations are lifted and independent of chemical potential. At high temperatures, the SQUID oscillations revert to conventional behaviour, ruling out asymmetry. The node lifting of the SQUID oscillations is consistent with low energy Andreev bound states exhibiting a nonsinusoidal current phase relation, coexisting with states possessing a conventional sinusoidal current phase relation. However, the finite nodal currents in the single junction diffraction pattern suggest an anomalous contribution to the supercurrent possibly carried by Majorana bound states, although we also consider the possibility of inhomogeneity.

cond-mat.mes-hall↗

arXiv2015

Tunable negative permeability in a three-dimensional superconducting metamaterial

We report on highly tunable radio frequency (rf) characteristics of a low-loss and compact three dimensional (3D) metamaterial made of superconducting thin film spiral resonators. The rf transmission spectrum of a single element of the metamaterial shows a fundamental resonance peak at $\sim$24.95 MHz that shifts to a 25$\%$ smaller frequency and becomes degenerate when a 3D array of such elements is created. The metamaterial shows an $\emph{in-situ}$ tunable narrow frequency band in which the real part of the effective permeability is negative over a wide range of temperature, which reverts to gradually near-zero and positive values as the superconducting critical temperature is approached. This metamaterial can be used for increasing power transfer efficiency and tunability of electrically small rf-antennas.

cond-mat.supr-con↗

arXiv2014

Phase Coherence and Andreev Reflection in Topological Insulator Devices

Topological insulators (TIs) have attracted immense interest because they host helical surface states. Protected by time-reversal symmetry, they are robust to non-magnetic disorder. When superconductivity is induced in these helical states, they are predicted to emulate p-wave pairing symmetry, with Majorana states bound to vortices. Majorana bound states possess non-Abelian exchange statistics which can be probed through interferometry. Here, we take a significant step towards Majorana interferometry by observing pronounced Fabry-Perot oscillations in a TI sandwiched between a superconducting and normal lead. For energies below the superconducting gap, we observe a doubling in the frequency of the oscillations, arising from the additional phase accumulated from Andreev reflection. When a magnetic field is applied perpendicular to the TI surface, a number of very sharp and gate-tunable conductance peaks appear at or near zero energy, which has consequences for interpreting spectroscopic probes of Majorana fermions. Our results demonstrate that TIs are a promising platform for exploring phase-coherent transport in a solid-state system.

cond-mat.mes-hall↗

arXiv2014

Dynamical Gate Tunable Supercurrents in Topological Josephson Junctions

Josephson junctions made of closely-spaced conventional superconductors on the surface of 3D topological insulators have been proposed to host Andreev bound states (ABSs) which can include Majorana fermions. Here, we present an extensive study of the supercurrent carried by low energy ABSs in Nb/Bi$_2$Se$_3$/Nb Josephson junctions in various SQUIDs as we modulate the carrier density in the Bi$_2$Se$_3$ barriers through electrostatic top gates. As previously reported, we find a precipitous drop in the Josephson current at a critical value of the voltage applied to the top gate. This drop has been attributed to a transition where the topologically trivial 2DEG at the surface is nearly depleted, causing a shift in the spatial location and change in nature of the helical surface states. We present measurements that support this picture by revealing qualitative changes in the temperature and magnetic field dependence of the critical current across this transition. In particular, we observe pronounced fluctuations in the critical current near total depletion of the 2DEG that demonstrate the dynamical nature of the supercurrent transport through topological low energy ABSs.

cond-mat.supr-con↗

arXiv2013

Plasmonic Scaling of Superconducting Metamaterials

Superconducting metamaterials are utilized to study the approach to the plasmonic limit simply by tuning temperature to modify the superfluid density, and thus the superfluid plasma frequency. We examine the persistence of artificial magnetism in a metamaterial made with superconductors in the plasmonic limit, and compare to the electromagnetic behavior of normal metals as a function of frequency as the plasma frequency is approached from below. Spiral-shaped Nb thin film meta-atoms of scaled dimensions are employed to explore the plasmonic behavior in these superconducting metamaterials, and the scaling condition allows extraction of the temperature dependent superfluid density, which is found to be in good agreement with expectations.

cond-mat.supr-con↗

arXiv2013

Imaging the Anisotropic Nonlinear Meissner Effect in Nodal YBa$_{2}$Cu$_{3}$O$_{7-δ}$ Thin-Film Superconductors

We have directly imaged the anisotropic nonlinear Meissner effect in an unconventional superconductor through the nonlinear electrodynamic response of both (bulk) gap nodes and (surface) Andreev bound states. A superconducting thin film is patterned into a compact self-resonant spiral structure, excited near resonance in the radio-frequency range, and scanned with a focused laser beam perturbation. At low temperatures, direction-dependent nonlinearities in the reactive and resistive properties of the resonator create photoresponse that maps out the directions of nodes, or of bound states associated with these nodes, on the Fermi surface of the superconductor. The method is demonstrated on the nodal superconductor YBa$_{2}$Cu$_{3}$O$_{7-δ}$ and the results are consistent with theoretical predictions for the bulk and surface contributions.

cond-mat.supr-con↗

arXiv2011

Superconducting RF Metamaterials Made with Magnetically Active Planar Spirals

Superconducting metamaterials combine the advantages of low-loss, large inductance (with the addition of kinetic inductance), and extreme tunability compared to their normal metal counterparts. Therefore, they allow realization of compact designs operating at low frequencies. We have recently developed radio frequency (RF) metamaterials with a high loaded quality factor and an electrical size as small as $\sim$$λ$658, ($λ$ is the free space wavelength) by using Nb thin films. The RF metamaterial is composed of truly planar spirals patterned with lithographic techniques. Linear transmission characteristics of these metamaterials show robust Lorentzian resonant peaks in the sub- 100 MHz frequency range below the $T_c$ of Nb. Though Nb is a non-magnetic material, the circulating currents in the spirals generated by RF signals produce a strong magnetic response, which can be tuned sensitively either by temperature or magnetic field thanks to the superconducting nature of the design. We have also observed strong nonlinearity and meta-stable jumps in the transmission data with increasing RF input power until the Nb is driven into the normal state. We discuss the factors modifying the induced magnetic response from single and 1-D arrays of spirals in the light of numerical simulations.

cond-mat.supr-con↗

arXiv2010

Rebuttal to "Comment by V.M. Krasnov on 'Counterintuitive consequence of heating in strongly-driven intrinsic junctions of Bi2Sr2CaCu2O8+d Mesas' "

In our article [1], we found that with increasing dissipation there is a clear, systematic shift and sharpening of the conductance peak along with the disappearance of the higher-bias dip/hump features (DHF), for a stack of intrinsic Josephson junctions (IJJs) of intercalated Bi2Sr2CaCu2O8+δ (Bi2212). Our work agrees with Zhu et al [2] on unintercalated, pristine Bi2212, as both studies show the same systematic changes with dissipation. The broader peaks found with reduced dissipation [1,2] are consistent with broad peaks in the density-of-states (DOS) found among scanning tunneling spectroscopy [3] (STS), mechanical contact tunneling [4] (MCT) and inferred from angle (momentum) resolved photoemission spectroscopy [5] (ARPES); results that could not be ignored. Thus, sharp peaks are extrinsic and cannot correspond to the superconducting DOS. We suggested that the commonality of the sharp peaks in our conductance data, which is demonstrably shown to be heating-dominated, and the peaks of previous intrinsic tunneling spectroscopy (ITS) data implies that these ITS reports might need reinterpretation.

cond-mat.supr-con↗

arXiv2009

Counterintuitive Consequence of Heating in Strongly-Driven Intrinsic-Junctions of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ Mesas

Anomalously high and sharp peaks in the conductance of intrinsic Josephson junctions in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ (Bi2212) mesas have been universally interpreted as superconducting energy gaps, but here we show they are a result of heating. This interpretation follows from a direct comparison to the equilibrium gap, $\mathit Δ$, measured in break junctions on similar Bi2212 crystals. As the dissipated power increases with a greater number of junctions in the mesa, the conductance peak abruptly sharpens and its voltage decreases to well below 2$\mathit Δ$. This sharpening, found in our experimental data, defies conventional intuition of heating effects on tunneling spectra, but it can be understood as an instability into a nonequilibrium two-phase coexistent state. The measured peak positions occur accurately within the voltage range that an S-shaped backbending is found in the {\it calculated} current-voltage curves for spatially {\it uniform} self-heating and that S-shape implies the potential for the uniform state to be unstable.

cond-mat.supr-con↗

arXiv2009

Emission of Terahertz Waves from Stacks of Intrinsic Josephson Junctions

By patterning mesoscopic crystals of Bi2Sr2CaCu2O8 (BSCCO) into electromagnetic resonators the oscillations of a large number of intrinsic Josephson junctions can be synchronized into a macroscopic coherent state accompanied by the emission of strong continuous wave THz-radiation. The temperature dependence of the emission is governed by the interplay of self-heating in the resonator and by re-trapping of intrinsic Josephson junctions which can yield a strongly nonmonotonic temperature dependence of the emission power. Furthermore, proper shaping of the resonators yields THzsources with voltage-tunable emission frequencies.

cond-mat.supr-con↗

arXiv2009

Thermal Management in Large Bi2212 Mesas used for Terahertz Sources

We present a thermal analysis of a patterned mesa on a Bi2Sr2CaCu2O8 (Bi2212) single crystal that is based on tunneling characteristics of the c-axis stack of ~800 intrinsic Josephson junctions in the mesa. Despite the large mesa volume (e.g., 40x300x1.2 micron3) and power dissipation that result in selfheating and backbending of the current-voltage curve (I-V), there are accessible bias conditions for which significant polarized THz-wave emission can be observed. We estimate the mesa temperature by equating the quasiparticle resistance, Rqp(T), to the ratio V/I over the entire I-V including the backbending region. These temperatures are used to predict the unpolarized black-body radiation reaching our bolometer and there is substantial agreement over the entire I-V. As such, backbending results from the particular Rqp(T) for Bi2212, as first discussed by Fenton, rather than a significant suppression of the energy gap. This model also correctly predicts the observed disappearance of backbending above ~60 K.

cond-mat.supr-con↗

arXiv2007

Evidence of Strong-Coupled Superconductivity in CaC6 from Tunneling Spectroscopy

Point-contact tunneling on CaC$_6$ crystals reproducibly reveals superconducting gaps, $Δ$, of 2.3$\pm$0.2 meV which are $\sim$~40% larger than earlier reports. That puts CaC$_6$ into the class of very strong-coupled superconductors since 2$Δ$/kT$_c\sim$~4.6. Thus soft Ca phonons will be primarily involved in the superconductivity, a conclusion that explains the large Ca isotope effect found recently for CaC$_6$. Consistency among superconductor-insulator-normal metal (SIN), SIS and Andreev reflection (SN) junctions reinforces the intrinsic nature of this result.

cond-mat.supr-con↗

arXiv2007

Investigation of the Tunneling Spectra in HgBr2-Intercalated Bi-2212 Single Crystals below and above Tc

Interlayer tunneling spectroscopy measurements were performed on mesa arrays of Bi-2212 single crystals, intercalated with HgBr$_2$. Tunneling conductances were obtained over a wide temperature range to examine the spectral features, especially the behavior of the quasiparticle peaks corresponding to superconducting energy gaps (SGs). Experimental spectra showed that gap-like features are still present even for the temperatures far above the transition temperature, T$_c$. This evidence is consistent with the idea that the SG evolves into a pseudogap above T$_c$ for HgBr$_2$-intercalated Bi-2212 single crystals.

cond-mat.supr-con↗