arXiv subjects

Marc Scheffler

Publications and source records attributed to Marc Scheffler.

At least 19 recordsLinked to original sources

arXiv2026

Evolution of electron spin resonance through a metallic quantum critical phase diagram

In the heavy-fermion metal YbRh2Si2, quantum criticality at a suppressed antiferromagnetic order is governed by the interplay of local magnetic moments and itinerant conduction electrons. We demonstrate how this can be investigated by a new experimental approach that enables the observation of electron spin resonance (ESR) across a broad range of frequencies and fields at very low temperatures. This allowed us to cover a large part of the phase diagram from the paramagnetic Fermi-liquid phase to the phase with antiferromagnetic order and including the quantum-critical regime. Both the ESR g-factor and the linewidth present distinct behaviors in these three regimes, providing further insight into the physics across a quantum critical point. Notably, when cooling down at a field directly towards the quantum critical point, both g-factor and linewidth continuously decrease. Furthermore, we observe a very good matching of the g-factor behavior upon field-tuning and temperature-tuning towards the quantum-critical point. We analyze and discuss the results in the context of present theories on ESR in strongly correlated electron systems.

cond-mat.str-el↗

arXiv2026

Contactless terahertz mapping of wafer-scale superconducting NbTiN thin films

For large-scale superconducting quantum technology, e.g. quantum computing, the homogeneity of wafer-scale superconducting thin films is vital for consistent performance of the fabricated devices. Terahertz (THz) spectroscopy as a contactless and non-destructive measurement technique is a powerful tool to characterize the superconducting films. In this work, a set of niobium titanium nitride (NbTiN) thin films on 4-inch and 6-inch silicon wafers, grown via plasma-enhanced magnetron sputtering, are investigated via THz spectroscopy: full wafers are mapped at room temperatures and exemplary segments are characterized at cryogenic temperatures. The deviations in observed sheet resistance depend on the used deposition device and the film thickness. While the deviations in superconducting sheet kinetic inductance match those of the normal-state sheet resistance, the critical temperature and energy gap exhibit little variation. This THz mapping technique demonstrates the feasibility of evaluating wafer-scale superconducting thin films before lithography, facilitating preparation of the thin films for reproducible device fabrication.

cond-mat.supr-con↗

arXiv2026

Signatures of Dynes superconductivity in the THz response of ALD-grown NbN thin films

The frequency-dependent complex optical conductivity reflects key properties of superconductors, such as the energy gap in the density of states (DOS) and the superfluid density. For disordered superconductors, the optical conductivity often can be described within Bardeen-Cooper-Schrieffer (BCS) theory, while in corresponding tunneling experiments, deviations in the observed DOS typically require modelling by the phenomenological Dynes formula. The implications of such Dynes DOS for optics were rarely discussed so far. Here we probe the terahertz conductivity of superconducting NbN thin films with thicknesses ranging from 4.5 to 20nm, which were grown by atomic layer deposition (ALD). Our frequency range from 0.3 to 2.1 THz covers energies below and above the spectral gap. For 20nm thick NbN, we find in the optical conductivity distinct deviations from the BCS model, including a step-like characteristic in the absorption at half the zero-temperature spectral gap. These observations can be fully captured by Dynes electrodynamics with a small and temperature-independent pair-breaking rate. For the other films, we also observe signs of Dynes electrodynamics, and we discuss the evolution of the energy gap, the superfluid density, and the pair-breaking rate as function of film thickness.

cond-mat.supr-con↗

arXiv2025

THz electrodynamics and superconducting energy scales of ZrN thin films

The terahertz (THz) properties of ZrN thin films grown with CMOS-techniques on industry-standard 300 mm silicon wafers are investigated in order to explore their superconducting behavior. The films have thicknesses ranging from 18 to 48 nm, and their critical temperatures Tc are between 5 and 7.3 K. We probe the real and imaginary parts of the complex dynamical conductivity sigma in the frequency range from 100 - 540 GHz (0.4 - 2.2 meV) and as a function of temperature. The experiments provide direct access to the low-energy electrodynamics and key materials parameters such as superconducting energy gap and superfluid density. Our findings indicate that ZrN is a weakly coupled BCS-type superconductor with a gap-to-Tc ratio of approximately 3.4 in the thick film limit. For thinner films, this coupling ratio increases up to 4.0, departing from the BCS prediction. The results establish large-scale ZrN thin films as promising material for high-frequency superconducting applications.

cond-mat.supr-con↗

arXiv2025

Probing Vortex Dynamics in 2D Superconductors with Scanning Quantum Microscope

The visualization of the magnetic responses of a two-dimensional (2D) superconducting material on the nanoscale is a powerful approach to unravel the underlying supercurrent behavior and to investigate critical phenomena in reduced dimensions. In this study, scanning quantum microscopy is utilized to explore the local magnetic response of the 2D superconductor 2H-NbSe2. Our technique enables both static and dynamic sensing of superconducting vortices with high sensitivity and a spatial resolution down to 30 nm, unveiling unexpected phenomena linked to the intrinsic 2D nature of the superconductor, which are challenging to detect with more conventional local probes. Vortices do not arrange in a hexagonal lattice, but form a distorted vortex glass with expanding vortex size. A vortex can exhibit strong local dynamics due to thermal excitation. As the critical temperature is approached, a clear melting of the vortex glass is identified, leading to distinct configurations under different cooling conditions. Vortex fluctuations can also be probed through spin Hahn-echo measurements, which reveal the spin decoherence even well below the critical temperature -- and, intriguingly, enhanced decoherence at lower temperatures. Spatiotemporal microscopy of the magnetic dynamics associated with vortex excitations and fluctuations provides direct evidence of 2D superconducting phenomena at the nanoscale.

cond-mat.supr-con↗

arXiv2025

High quality superconducting tantalum resonators with beta phase defects

For practical superconducting quantum processors, orders of magnitude improvement in coherence is required, motivating efforts to optimize hardware design and explore new materials. Among the latter, the coherence of superconducting transmon qubits has been shown to improve by forming the qubit capacitor pads from $\alpha$-tantalum, avoiding the meta-stable $\beta$-phase that forms when depositing tantalum at room temperature, and has been previously identified to be a source of microwave losses. In this work, we show lumped element resonators containing $\beta$-phase tantalum in the form of inclusions near the metal-substrate interface with internal quality factors ($Q_\text{i}$) up to $(5.0 \pm 2.5) \times 10^6$ in the single photon regime. They outperform resonators with no sign of the $\beta$-phase in x-ray diffraction and thermal quasi-particle loss. Our results indicate that small concentrations of $\beta$-phase can be beneficial, enhancing critical magnetic fields and potentially, for improving coherence in tantalum based superconducting circuits.

quant-ph↗

arXiv2025

Influence of growth parameters on the superconducting transition temperature in granular aluminum films

This study investigates the influence of various growth parameters on normal-state resistivity and superconducting transition temperature Tc of granular aluminum films. Specifically, we focus on the effects of oxygen flow and aluminum evaporation rate during the growth process conducted at different substrate temperatures, from 300 K down to 25 K. We report systematic correlations between the growth conditions, the normal-state resistivity, and Tc. Furthermore our findings provide insights into optimizing the superconducting characteristics of granular aluminum.

cond-mat.supr-con↗

arXiv2024

Interplay of coupling, residual, and quasiparticle losses for the frequency- and temperature-dependent quality factor of superconducting resonators

The overall, loaded quality factor $Q_\mathrm{L}$ quantifies the loss of energy stored in a resonator. Here we discuss on general grounds how $Q_\mathrm{L}$ of a planar microwave resonator made of a conventional superconductor should depend on temperature and frequency. We consider contributions to $Q_\mathrm{L}$ due to dissipation by thermal quasiparticles ($Q_\mathrm{QP}$), due to residual dissipation ($Q_\mathrm{Res}$), and due to coupling ($Q_\mathrm{C}$). We present experimental data obtained with superconducting stripline resonators fabricated from lead (Pb), with different center conductor widths and different coupling gaps. We probe the resonators at various harmonics between 0.7 GHz and 6 GHz and at temperatures between 1.5 K and 7 K. We find a strongly frequency- and temperature-dependent $Q_\mathrm{L}$, which we can describe by a lumped-element model. For certain resonators at lowest temperatures we observe a maximum in the frequency-dependent $Q_\mathrm{L}$ when $Q_\mathrm{Res}$ and $Q_\mathrm{C}$ match, and here the measured $Q_\mathrm{L}$ can exceed $2\times 10^5$.

cond-mat.supr-con↗

arXiv2024

Gapped magnetic ground state in the spin-liquid candidate $\kappa$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$ suggested by magnetic spectroscopy

The nature of the magnetic ground state of highly frustrated systems remained puzzling to this day. Here, we have performed multifrequency electron spin resonance (ESR) measurements on a putative quantum spin liquid compound $\kappa$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$, which is a rare example of $S = 1/2$ spins on a triangular lattice. At high temperatures, the spin susceptibility exhibits a weak temperature dependence which can be described by the Heisenberg model with an antiferromagnetic exchange interaction of strength $J/k_B \approx 175$ K. At low temperatures, however, the rapid drop of the static spin susceptibility, together with monotonic decrease of the ESR linewidth indicates that strong singlet correlations develop below a pairing energy scale $T^*$ accompanied by a spin gap. On the other hand, a weak Curie-like spin susceptibility and the angular dependence of the linewidth suggest additional contribution from impurity spins. We propose the gradual formation of spin singlets with an inhomogeneous spin gap at low temperatures.

cond-mat.str-el↗

arXiv2024

Tuning the superconducting dome in granular aluminum thin films

Granular aluminum, which consists of nanometer-sized aluminum grains separated by aluminum oxide, is a peculiar superconductor. Its phase diagram as function of normal-state resistivity features a superconducting dome with a maximum critical temperature Tc well above the Tc = 1.2 K of pure aluminum. Here we show how the maximum Tc of this superconducting dome grows if the substrate temperature during deposition is lowered from 300 K to cooling with liquid nitrogen (150 K and 100 K) and liquid helium (25 K). The highest Tc we observe is 3.27 K. These results highlight that granular aluminum is a model system for complex phase diagrams of superconductors and demonstrate its potential in the context of high kinetic inductance applications. This is augmented by our observation of comparably sharp superconducting transitions of high-resistivity samples grown at cryogenic temperatures and by a thickness dependence even for films substantially thicker than the grain size.

cond-mat.supr-con↗

arXiv2023

Characterization of harmonic modes and parasitic resonances in multi-mode superconducting coplanar resonators

Planar superconducting microwave transmission line resonators can be operated at multiple harmonic resonance frequencies. This allows covering wide spectral regimes with high sensitivity, as it is desired e.g. for cryogenic microwave spectroscopy. A common complication of such experiments is the presence of undesired 'spurious' additional resonances, which are due to standing waves within the resonator substrate or housing box. Identifying the nature of individual resonances ('designed' vs. 'spurious') can become challenging for higher frequencies or if elements with unknown material properties are included, as is common for microwave spectroscopy. Here we discuss various experimental strategies to distinguish designed and spurious modes in coplanar superconducting resonators that are operated in a broad frequency range up to 20 GHz. These strategies include tracking resonance evolution as a function of temperature, magnetic field, and microwave power. We also demonstrate that local modification of the resonator, by applying minute amounts of dielectric or ESR-active materials, lead to characteristic signatures in the various resonance modes, depending on the local strength of the electric or magnetic microwave fields.

cond-mat.supr-con↗

arXiv2020

Gapped magnetic ground state in quantum-spin-liquid candidate $\kappa$-(BEDT-TTF)$_2$-Cu$_2$(CN)$_3$

Geometrical frustration, quantum entanglement and disorder may prevent long-range order of localized spins with strong exchange interactions, resulting in a novel state of matter. $\kappa$-(BEDT-TTF)$_2$-Cu$_2$(CN)$_3$ is considered the best approximation of this elusive quantum-spin-liquid state, but its ground-state properties remain puzzling. Here we present a multi-frequency electron-spin resonance study down to millikelvin temperatures, revealing a rapid drop of the spin susceptibility at $T^*=6\,\mathrm{K}$. This opening of a spin gap, accompanied by structural modifications, suggests the enigmatic `$6\,\mathrm{K}$-anomaly' as the transition to a valence-bond-solid ground state. We identify an impurity contribution that becomes dominant when the intrinsic spins form singlets. Only probing the electrons directly manifests the pivotal role of defects for the low-energy properties of quantum-spin systems without magnetic order.

cond-mat.str-el↗

arXiv2020

Indium tin oxide films meet circular Rydberg atoms: prospects for novel quantum simulation schemes

Long-lived circular Rydberg atoms are picking up increasing interest for boosting coherence times in Rydberg-based quantum simulation. We elaborate a novel approach to stabilize circular Rydberg states against spontaneous and blackbody-induced decay using a suppression capacitor made from indium tin oxide (ITO) thin films, which combine reflection of microwaves with transparency in the visible spectral range. To this end, we perform detailed characterization of such films using complementary spectroscopic methods at GHz and THz frequencies and identify conditions that allow for reaching circular-state lifetimes up to tens of milliseconds in a room-temperature environment. We discuss prospects of our findings in view of the quest for quantum simulations with high-$n$ circular Rydberg states at room temperature.

physics.atom-ph↗

arXiv2020

Cryogenic frequency-domain electron spin resonance spectrometer based on coplanar waveguides and field modulation

We present an instrument to perform frequency-domain electron spin resonance (ESR) experiments that is based on coplanar waveguides and field modulation. A large parameter space in frequency (up to 25 GHz), magnetic field (up to 8 T), and temperature (down to 1.6 K) is accessible. We performed experiments on DPPH (2,2-diphenyl-1-picrylhydrazyl) as a standard to calibrate the field modulation as well as on a carbon fibre sample to estimate the overall sensitivity of the instruments. Spectra of a ruby sample in a broad frequency- and field range at cryogenic temperatures are recorded with and without field modulation. The comparison reveals the improved signal-to-noise ratio achieved by field modulation.

physics.ins-det↗

arXiv2019

Planar GHz Resonators on SrTiO3: Suppressed Losses at Temperatures below 1 K

The complex dielectric constant $\hat{\epsilon} = \epsilon_1 + i \epsilon_2$ of SrTiO$_3$ reaches high values $\epsilon_1 \approx 2*10^{4}$ at cryogenic temperatures, while the dielectric losses ($\epsilon_2$) are much stronger than for other crystalline dielectrics. SrTiO$_3$ is a common substrate for oxide thin films, like the superconducting LaAlO$_3$/SrTiO$_3$ system, but the large $\epsilon_1$ and $\epsilon_2$ restrict high-frequency quantum devices on SrTiO$_3$. Here we present superconducting coplanar Nb resonators on SrTiO$_3$, which we successfully operate in a distant-flip-chip geometry at frequencies that exceed 1 GHz. We find a pronounced and unexpected increase in resonator quality factor $Q$ at temperatures below 1 K, reaching up to $Q \approx 800$. We attribute this to substantial changes of the dielectric losses in SrTiO$_3$ at mK temperatures, and we also detect non-monotonous changes in the temperature-dependent $\epsilon_1$. These findings challenge our present understanding of the dielectric properties of SrTiO$_3$ and at the same time demonstrate that cryogenic high-frequency devices on SrTiO$_3$ are more feasible than previously assumed.

cond-mat.supr-con↗

arXiv2019

Microwave probing of bulk dielectrics using superconducting coplanar resonators in distant-flip-chip geometry

Dielectric measurements on insulating materials at cryogenic temperatures can be challenging, depending on the frequency and temperature ranges of interest. We present a technique to study the dielectric properties of bulk dielectrics at GHz frequencies. A superconducting coplanar Nb resonator is deposited directly on the material of interest, and this resonator is then probed in distant-flip-chip geometry with a microwave feedline on a separate chip. Evaluating several harmonics of the resonator gives access to various probing frequencies, in the present studies up to 20 GHz. We demonstrate the technique on three different materials (MgO, LaAlO3, and TiO2), at temperatures between 1.4 K and 7 K.

cond-mat.supr-con↗

arXiv2019

Characterizing dielectric properties of ultra-thin films using superconducting coplanar microwave resonators

We present an experimental approach for cryogenic dielectric measurements on ultra-thin insulating films. Based on a coplanar microwave waveguide design we implement superconducting quarter-wave resonators with inductive coupling, which allows us to determine the real part $\varepsilon_1$ of the dielectric function at GHz frequencies and for sample thicknesses down to a few nm. We perform simulations to optimize resonator coupling and sensitivity, and we demonstrate the possibility to quantify $\varepsilon_1$ with a conformal mapping technique in a wide sample-thickness and $\varepsilon_1$-regime. Experimentally we determine $\varepsilon_1$ for various thin-film samples (photoresist, MgF$_2$, and SiO$_2$) in the thickness regime of nm up to $\mu m$. We find good correspondence with nominative values and we identify the precision of the film thickness as our predominant error source. Additionally we present a temperature-dependent measurement for a SrTiO$_3$ bulk sample, using an in-situ reference method to compensate for the temperature dependence of the superconducting resonator properties.

cond-mat.mtrl-sci↗

arXiv2018

Role of non-linear effects and standing waves in microwave spectroscopy: Corbino measurements on superconductors and VO$_2$

Broadband microwave spectroscopy can probe material properties in wide spectral and temperature ranges, including superconductors at cryogenic temperatures. The quality of such measurements crucially depends on the calibration, which also removes from the obtained spectra signatures of standing waves. Here we consider low-temperature reflection measurements in Corbino geometry, and we show that the non-linear response of superconducting samples close to the critical temperature can lead to strong signatures of standing waves even in a well-calibrated Corbino spectrometer. We demonstrate our findings with microwave measurements as a function of frequency and temperature for a variety of superconducting samples and for different length of the microwave transmission line. Finally we show that such non-linear effects extend beyond the case of superconductors by probing a VO$_2$ thin film at the insulator-metal transition.

cond-mat.supr-con↗