The Superconducting Quantum Materials and Systems Center, a U. S. Department of Energy National Quantum Information Science Research Center, has conducted a comprehensive and coordinated study using superconducting transmon qubit chips with known performance metrics to identify the underlying materials-level sources of device-to-device performance variation. Following qubit coherence measurements, these qubits of varying base superconducting metals and substrates have been examined with various non-destructive and invasive material characterization techniques at Northwestern University, Ames National Laboratory, and Fermilab as part of a blind study. We find trends in variations of the depth of the etched substrate trench, the thickness of the surface oxide, and the geometry of the sidewall, which when combined, lead to correlations with the T1 lifetime across different qubits on the same chip. In addition, we provide a list of features that varied from device to device, for which the impact on performance requires further studies. Finally, we identify two low-temperature characterization techniques that may potentially serve as proxy tools for qubit measurements. These insights provide materials-oriented solutions to not only reduce performance variations across neighboring devices but also to engineer and fabricate devices with optimal geometries to achieve performance metrics beyond the state-of-the-art values.

We have synthesized layered superconducting LiNbO_2 crystals through a bulk phase transformation from LiNbO_3 single crystals via CaH_2 reduction. As the Nb valence is reduced from 5+ to 3+, the material undergoes a structural transformation to the resulting product, LiNbO_2, which is accompanied by metallic behavior and a superconducting transition, Tc onset, as high as 14.4 K. Secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) show that the resulting phase is hole-doped through de-lithiation during the reduction. Magnetization and AC susceptibility measurements from a tunnel diode resonator confirm the bulk nature of superconductivity with a superconducting volume fraction of approximately 77

We study the effects of flux creep on the linear AC response of the vortex lattice in single crystals Ca3Ir4Sn13 by measuring the Campbell penetration depth, AC(T, H, t). Thermal fluctuations release vortices from shallow pinning sites, only for them to become re-trapped by deeper potential wells, causing an initial increase of the effective Labusch parameter, which is proportional to the pinning well curvature. This effect cannot be detected in conventional magnetic relaxation measurements but is revealed by our observation of a nonmonotonic time evolution of AC(T, H, t), which directly probes the average curvature of the occupied pinning centers. The time evolution of AC(T, H, t) was measured at different temperatures in samples with different densities of pinning centers produced by electron irradiation. The curves can be collapsed together when plotted on a logarithmic time scale t -> T ln (t/t0) confirming that the time evolution is driven by flux creep. The AC(T, H, t) is hysteretic with a noticeable nonmonotonic relaxation in the presence of a vortex density gradient (after zero-field cooling), but is monotonic after field cooling, where the vortex density is uniform. This result quantitatively corroborates the novel picture of vortex creep based on the strong pinning theory.

London, lambda(L)(T), and Campbell, lambda(C)(T), penetration depths were measured in single crystals of a topological superconductor candidate AuSn4. At low temperatures, lambda(L)(T) is exponentially attenuated and, if fitted with the power law, lambda(T)similar to T-n, gives exponents n>4n>4, indistinguishable from the isotropic single s-s-wave gap Bardeen-Cooper-Schrieffer (BCS) asymptotic. The superfluid density fits perfectly in the entire temperature range to the BCS theory. The superconducting transition temperature, T-c=2.40 +/- 0.05K, does not change after 2.5 MeV electron irradiation, indicating the validity of the Anderson theorem for isotropic s-s-wave superconductors. Campbell penetration depth before and after electron irradiation shows no hysteresis between the zero-field cooling (ZFC) and field cooling (FC) protocols, consistent with the parabolic pinning potential. Interestingly, the critical current density estimated from the original Campbell theory decreases after irradiation, implying that a more sophisticated theory involving collective effects is needed to describe vortex pinning in this system. In general, our thermodynamic measurements strongly suggest that the bulk response of the AuSn4 crystals is fully consistent with the isotropic s-wave weak-coupling BCS superconductivity.

We present nuclear magnetic resonance data in BaFe2As2 in the presence of pulsed strain fields that are interleaved in time with the radio frequency excitation pulses. In this approach, the preceding nuclear magnetization acquires a phase shift that is proportional to the strain and pulse time. The sensitivity of this approach is limited by the homogeneous decoherence time, T2, rather than the inhomogeneous linewidth. We measure the nematic susceptibility as a function of temperature and demonstrate a three orders of magnitude improvement in sensitivity. This approach will enable studies of the strain response in a broad range of materials that previously were inaccessible due to inhomogeneous broadening.

London and Campbell penetration depths were measured in single crystals of the endohedral gallide cluster superconductor, Mo8Ga41. The full temperature range superfluid density, rho s(T), is consistent with the clean isotropic s-wave weak-coupling BCS theory without any signs of the second gap or strong coupling. The temperature dependence of the Campbell length is hysteretic between zero-field cooling (ZFC) and field-cooling (FC) protocols, indicating an anharmonic vortex pinning potential. The field dependence of the effective critical current density, jc (H), reveals an unusual result. While in the ZFC protocol, jc (H) is monotonically suppressed by the magnetic field, it exhibits a profound "hidden" peak effect in the FC protocol, that is, without a vortex density gradient. We suggest a possible novel mechanism for such a peak effect, which involves both static and dynamic aspects of vortex pinning.

A recent theory of the disorder-dependent slope of the upper critical field at the superconducting transition temperature T-c, defined as S equivalent to|dH(c2)/dT|(T -> Tc), is extended to multiband superconductors aimed at iron-based super conductors, considering two constant gaps of different magnitudes and, potentially, different signs. We show that the slope S decreases with increasing nonmagnetic scattering rate P in the s(+/-) pairing state and increases in the s(++ )superconductor for a reasonable range of parameters. The experiment shows that in a typical iron-based superconductor, Ba(1-x)KxFe(2)As(2)(BaK122), the nonmagnetic disorder induced by electron irradiation leads to an increasing S(P) across the superconducting "dome," at differentx. This implies that Ba(1-x)KxFe(2)As(2 )is likely an s(++)superconductor with two effective gaps of different magnitudes, at least at moderate doping levels, x<0.6.This work reopens a decade-long discussion about the nature of the superconducting order parameter in ironpnictides.

We report an unusual anisotropic paramagnetic peak effect observed in reversible magnetization of a single crystalline nodal superconductor Rh_17S_15. Both temperature- and field-dependent magnetization measurements reveal a distinct novel vortex state above approximately 1 T. This peak effect is most pronounced when the magnetic field, H, is applied parallel to the [111] direction, whereas it diminishes for H∥[110]. Intriguingly, for H∥[100], instead of a peak, we observe a step-like decrease in M(T), with the step amplitude increasing in larger applied magnetic fields. This behavior is opposite to the expectations of conventional Meissner expulsion. The magnitude of the peak effect, expressed in terms of dimensionless volume susceptibility, is on the order of Δχ=10^-5 (with full diamagnetic screening corresponding to χ=-1). The observed anisotropic paramagnetic vortex response is unusual considering the cubic symmetry of Rh_17S_15. We propose that in this distinct vortex phase, a small but finite attractive interaction between vortices below H_c2 may be responsible for this unusual phenomenon. Furthermore, the vortices seem to prefer aligning along the [111] direction, rotating toward it when the magnetic field is applied in other directions. Our findings add another item to the list of unusual properties of Rh_17S_15 that attracted recent attention as the first unconventional superconductor that has a mineral analog, miassite, found in nature.

The effects of 2.5 MeV electron irradiation were studied in the superconducting phase of single crystals of LaNiGa$_2$, using measurements of electrical transport and radio-frequency magnetic susceptibility. The London penetration depth is found to vary exponentially with temperature, suggesting a fully gapped Fermi surface. The inferred superfluid density is close to that of a single-gap weak-coupling isotropic $s-$wave superconductor. Superconductivity is extremely robust against nonmagnetic point-like disorder induced by electron irradiation. Our results place strong constraints on the previously proposed triplet pairing state by requiring fine-tuned impurity scattering amplitudes and are most naturally explained by a sign-preserving, weak-coupling, and approximately momentum independent singlet superconducting state in LaNiGa$_2$, which does not break time-reversal symmetry. We discuss how our findings could be reconciled with previous measurements indicating magnetic moments in the superconducting phase.

The coexistence and competition between the charge density wave (CDW) and superconductivity was studied by varying the Rh/Ir ratio. The superconducting transition temperature, $T_c$, varies from 7 K in pure Ir ($x=0$) to 8.3 K in pure Rh ($x=1$). Temperature-dependent electrical resistivity reveals monotonic suppression of the CDW transition temperature, $T_{\text{CDW}}(x)$. The CDW starts in pure Ir, $x=0$, at $T_{\text{CDW}}\approx40$~K and extrapolates roughly linearly to zero at $x_c \approx 0.53-0.58$ under the superconducting dome. Magnetization and transport measurements show a significant influence of CDW on superconducting and normal states. Meissner expulsion is substantially reduced in the CDW region, indicating competition between the CDW and superconductivity. The low-temperature resistivity is higher in the CDW part of the phase diagram, consistent with the reduced density of states due to CDW gapping. Its temperature dependence just above $T_c$ shows signs of non-Fermi liquid behavior in a cone-like composition pattern. We conclude that the $\text{Ca}_3(\text{Ir}_{1-x}\text{Rh}_x)_4\text{Sn}_{13}$ alloy is a good candidate for a composition-driven quantum critical point (QCP) at ambient pressure. Temperature-dependent electrical resistivity reveals monotonic suppression of the CDW transition temperature, $T_{\text{CDW}}(x)$. The CDW starts in pure Ir, $x=0$, at $T_{\text{CDW}}\approx40$~K and extrapolates roughly linearly to zero at $x_c \approx 0.53-0.58$ under the superconducting dome. Magnetization and transport measurements show a significant influence of CDW on superconducting and normal states. Meissner expulsion is substantially reduced in the CDW region, indicating the competition between the CDW and superconductivity. The low-temperature resistivity is higher in the CDW part of the phase diagram, consistent with the reduced density of states due to CDW gapping. Its temperature dependence just above $T_c$ shows clear signs of non-Fermi-liquid behavior in a cone-like composition pattern. We conclude that the $\text{Ca}_3(\text{Ir}_{1-x}\text{Rh}_x)_4\text{Sn}_{13}$ alloy is a good candidate for a composition-driven quantum critical point (QCP) at ambient pressure.

The quasi-skutterudites (Ca_xSr_1-x)_3(Rh, Ir)_4Sn_13 show a rare nonmagnetic quantum critical point associated with the second-order charge-density-wave (CDW) and structural distortion transition extended under the superconducting "dome". So far, the non-thermal tuning parameters for accessing the QCP included changing stoichiometry, pressure, and a magnetic field. Here we add another parameter – a nonmagnetic point-like disorder induced by 2.5 MeV electron irradiation. The non-Fermi liquid regime was inferred from the analysis of the temperature-dependent resistivity, ρ(T), in single crystals of (Ca_xSr_1-x)_3Rh_4Sn_13. Starting at compositions below the known QCP concentration of x_c=0.9, added disorder resulted in a progressively larger linear term and a reduced quadratic term in ρ(T). This behavior is supported by theoretical analysis based on the idea of superconducting fluctuations encompassing the crossover from quantum to thermal regimes. Our results strongly support the concept that the nonmagnetic disorder can drive the system toward the quantum critical regime.

London penetration depth was measured in niobium foils, thin films, single crystals, and superconducting radio-frequency (SRF) cavity pieces cut out from different places. The low-temperature (T<Tc/3) variation, sensitive to the low-energy quasiparticles with states inside the superconducting gap, differs dramatically between different types of samples. With the help of phenomenological modeling, we correlate these different behaviors with known pair-breaking mechanisms and show that such measurements may help distinguish between different pair-breaking mechanisms, such as niobium hydrides and two-level systems (TLS). The conclusions also apply to SRF cavities when tracking the temperature-dependent quality factor and the resonant frequency.

The intrinsically superconducting Dirac semimetal 2M-WS2 is a promising candidate for realizing proximityinduced topological superconductivity in its protected surface states. A precise characterization of the bulk superconducting state is essential to understand the nature of surface superconductivity in the system. Here, we report a detailed experimental study of the temperature -dependent London penetration depth, lambda(T ), the upper critical field, Hc2(T ), and the effects of nonmagnetic disorder on these quantities, as well as on the superconducting transition temperature Tc in single crystals of 2M-WS2. We observe a power -law variation of lambda(T) proportional to T3 at temperatures below 0.35Tc. Nonmagnetic pointlike disorder induced by 2.5 MeV electron irradiation at various doses results in a significant suppression of Tc. These observations are markedly different from expectations for a fully gapped isotropic s -wave superconductor. Together with the substantial increase of slope, dHc2/dT |T =Tc, with increasing disorder, our results suggest a strongly anisotropic s++ multiband superconducting state. These results have direct consequences for the expected proximity -induced superconductivity of the topological surface states.

The effect of annealing on the superconducting properties of niobium single crystals cut from the same master boule was studied by local and global magnetic measurements, as well as scanning tunneling microscopy (STM). The formation of large hydride precipitates was observed in unannealed samples. The variation in structural and magnetic properties was studied after annealing under high vacuum at 800 C, 1400 C, and near the melting point of niobium (2477 C) for a few seconds. The initial samples had a high hydrogen content. Polarized optics and magneto-optical studies show that the formation of large niobium hydride precipitates is suppressed already by 800 C annealing. However, the overall superconducting properties in the annealed samples did not improve after annealing, and in fact, worsened. The superconducting transition temperature decreased, the upper critical field increased, and the pinning strength increased. Parallel studies were conducted using STM, where the sample was annealed initially at 400 C, measured, annealed again at 1700 C, and measured again. These studies revealed a ``dirty'' superconducting gap with a significant spatial variation of tunneling conductance after annealing at 400 C. The clean gap was recovered after annealing at 1700 C. It is likely that these results are due to oxygen redistribution near the surface, which is always covered by oxide layers in as-grown crystals. Overall, the results indicate that vacuum annealing at least up to 1400 C, while expected to remove a large amount of hydrogen, introduces additional nanosized defects, perhaps hydride precipitates, that act as efficient pair-breaking and pinning centers.

Single crystals of the quasi-skutterudite compounds Ca3(Ir1-xRhx)4Sn13(3-4-13) were synthesized by flux growth and characterized by x-ray diffraction, energy dispersive x-ray spectroscopy, magnetization, resistivity, and radio frequency magnetic susceptibility techniques. The coexistence and competition between the charge density wave (CDW) and superconductivity was studied by varying the Rh/Ir ratio. The superconducting transition temperature,Tc, varies from 7 K in pure Ir (x = 0) to 8.3 K in pure Rh (x = 1). Temperature-dependent electrical resistivity reveals monotonic suppression of the CDW transition temperature,TCDW(x). The CDW starts in pure Ir,x = 0, atTCDW≈ 40 K and extrapolates roughly linearly to zero atxc≈0.53-0.58 under the superconducting dome. Magnetization and transport measurements show a significant influence of CDW on superconducting and normal states. Meissner expulsion is substantially reduced in the CDW region, indicating competition between the CDW and superconductivity. The low-temperature resistivity is higher in the CDW part of the phase diagram, consistent with the reduced density of states due to CDW gapping. Its temperature dependence just aboveTcshows signs of non-Fermi liquid behavior in a cone-like composition pattern. We conclude that the Ca3(Ir1-xRhx)4Sn13alloy is a good candidate for a composition-driven quantum critical point at ambient pressure.

LaCrGe$_3$ has attracted attention as a potential candidate for studies of quantum phase transitions in a ferromagnetic material. The application of pressure avoids a quantum critical point by developing a new magnetic phase. It was suggested that the disorder may provide an alternative route to a quantum critical point. We used low-temperature 2.5 MeV electron irradiation to induce relatively small amounts of point-like disorder in single crystals of LaCrGe$_3$. Irradiation leads to an increase of the resistivity at all temperatures with some deviation from the Matthiessen rule. Hall effect measurements show that electron irradiation does not cause any detectable change in the carrier density. Unexpectedly, the Curie temperature, $T_{\text{FM}}$, \emph{increases} with the increase of disorder from approximately 90 K in pristine samples up to nearly 100 K in the heavily irradiated sample, with a tendency towards saturation at higher doses. Although the mechanism of this effect is not entirely clear, we conclude that it cannot be caused by effective ``doping" or ``pressure" due to electron irradiation. We suggest that disorder-induced broadening of a sharp peak in the density of states, $D(E)$, situated at $E_p=E_F-0.25$ eV below the Fermi energy, $E_F$, causes an increase in $D(E_F)$, leading to an enhancement of $T_\text{FM}$ in this itinerant ferromagnet.

London and Campbell penetration depths were measured in single crystals of the endohedral gallide cluster superconductor, Mo_8Ga_41. The full temperature range superfluid density is consistent with the clean isotropic s-wave weak-coupling BCS theory without any signs of the second gap or strong coupling. The temperature dependence of the Campbell length is hysteretic between zero-field cooling (ZFC) and field-cooling (FC) protocols, indicating an anharmonic vortex pinning potential. The field dependence of the effective critical current density, j_c(H), reveals an unusual result. While in the ZFC protocol, j_c(H) is monotonically suppressed by the magnetic field, it exhibits a profound “hidden” peak effect in the FC protocol, that is, without a vortex density gradient. We suggest a possible novel mechanism for the formation of the peak effect, which involves both static and dynamic aspects.

Solid state chemistry has produced a plethora of materials with properties not found in nature. For example, high-temperature superconductivity in cuprates is drastically different from the superconductivity of naturally occurring metals and alloys and is frequently referred to as unconventional. Unconventional superconductivity is also found in other synthetic compounds, such as iron-based and heavy-fermion superconductors. Here, we report compelling evidence of unconventional nodal superconductivity in synthetic samples of Rh 17 S 15 ( T c = 5.4 K), which is also found in nature as the mineral miassite. We investigated the temperature-dependent variation of the London penetration depth Δ λ ( T ) and the disorder evolution of the critical superconducting temperature T c and the upper critical field H c 2 ( T ) in single crystalline Rh 17 S 15 . We found a T − linear temperature variation of Δ λ ( T ) below 0.3 T c , which is consistent with the presence of nodal lines in the superconducting gap of Rh 17 S 15 . The nodal character of the superconducting state is supported by the observed suppression of T c and H c 2 ( T ) in samples with a controlled level of non-magnetic disorder introduced by 2.5 MeV electron irradiation. We propose a nodal sign-changing superconducting gap in the A 1 g irreducible representation, which preserves the cubic symmetry of the crystal and is in excellent agreement with the derived superfluid density. To the best of our knowledge, this establishes miassite as the only mineral known so far that reveals unconventional superconductivity in its clean synthetic form, though it is unlikely that it is present in natural crystals because of unavoidable impurities that quickly destroy nodal superconductivity.

The effects of 2.5-MeV electron irradiation on the magnetic properties of single crystals of the Remeika series superconductor Ca3Rh4Sn13 were studied using high-frequency ac susceptometry, magnetization, and electrical transport. This low-pinning cubic stannide is an ideal system to examine the effects of a controlled nonmagnetic pointlike disorder. The measured Campbell penetration depth was used to extract the magnetic field dependence of the unrelaxed critical current density, j(c)(H). The critical current is a monotonic function of a magnetic field in pristine state. However, even the lowest dose of electron irradiation causes a pronounced peak effect in j(c)(H). The peak effect is also observed in magnetization measurements performed with different characteristic time windows. We conclude that additional defects trigger the appearance of a disordered vortex phase at magnetic fields close to the upper critical field, and the peak effect is the result of a crossover from the weakly distorted low-field vortex lattice to the disordered high-field vortex phase. These results strongly support the static picture of the peak effect formation in Ca3Rh4Sn13 in which this is a feature of the critical current density, j(c)(H), and not the result of magnetic field-dependent vortex relaxation, j(H, t).

Members of the CaK(Fe1-xCrx)(4)As-4 series have been synthesized by high-temperature solution growth in single-crystalline form and characterized by x-ray diffraction, elemental analysis, and magnetic and transport measurements. The effects of Cr substitution on the superconducting and magnetic ground states of CaKFe4As4 (T-c = 35 K) have been studied. These measurements show that the superconducting transition temperature decreases monotonically and is finally suppressed below 1.8 K as x is increased from 0 to 0.038. For x-values greater than 0.012, signatures of a magnetic transition can be detected in magnetic measurements with the associated features in the transport measurements becoming detectable for x >= 0.038. The magnetic transition temperature increases in a roughly linear manner as Cr substitution increases. A temperature-composition (T-x) phase diagram is constructed, revealing a half-dome of superconductivity with the magnetic transition temperature, T*, appearing near 22 K for x similar to 0.017 and rising slowly up to 60 K for x similar to 0.077. The T-x phase diagrams for CaK(Fe1-xTx)(4)As-4 for T = Cr and Mn are essentially the same despite the nominally different band filling; this is in marked contrast to T = Co and Ni series for which the T-x diagrams scale by a factor of 2, consistent with the different changes in band filling Co and Ni would produce when replacing Fe. Superconductivity of CaK(Fe1-xCrx)(4)As-4 is also studied as a function of magnetic field. A clear change in H-c2'(T)/T-c, where H-c2'(T) is dH(c2)(T)/dT, at x similar to 0.012 is observed and probably is related to a change of the Fermi surface due to magnetic order. Coherence length and the London penetration depths are also calculated based on H-c1 and H-c2 data. Both of them as a function of x show changes near x = 0.012, again consistent with Fermi surface changes associated with the magnetic ordering seen for higher x-values.