arXiv subjects

Felix Lang

Publications and source records attributed to Felix Lang.

11 recordsLinked to original sources

arXiv2026

Engineering a Bound State in the Continuum via Quantum Interference

Quantum mechanical interaction potentials typically support either localized bound states below the dissociation threshold or delocalized scattering states above it. While bound states are energetically isolated, scattering states embed a quantum system in a continuum of environmental modes, making dissipation and loss intrisic features of open quantum systems. A striking exception are bound states in the continuum (BICs), which remain localized despite lying within the scattering continuum due to destructive interference. It was predicted that such states can arise from the interference of two Feshbach resonances coupled to a common continuum, yet this mechanism has remained experimentally inaccessible in genuine quantum systems. Here we demonstrate the formation of such an interference-stabilized state in ultracold collisions of ${}^6$Li atoms by coherently coupling two tunable Feshbach resonances using Floquet engineering. At a critical parameter point, both elastic and inelastic coupling to the continuum vanish, yielding a molecular state above the dissociation threshold. Loss spectroscopy, quench dynamics, and rf-photoassociation directly reveal the resulting decoupling from scattering states. Our observations are quantitatively captured by full coupled-channel calculations and a minimal non-Hermitian model, identifying a Friedrich-Wintgen BIC. Our results establish quantum interference as a powerful mechanism for controlling openness in quantum matter and for engineering non-Hermitian Hamiltonians.

cond-mat.quant-gas↗

arXiv2025

Quantification of mobile ions in perovskite solar cells with thermally activated ion current measurements

Mobile ions play a key role in the degradation of perovskite solar cells, making their quantification essential for enhancing device stability. Various electrical measurements have been applied to characterize mobile ions. However, discerning between different ionic migration processes can be difficult. Furthermore, multiple measurements at different temperatures are usually required to probe different ions and their activation energies. Here, we demonstrate a new characterization technique based on measuring the thermally activated ion current (TAIC) of perovskite solar cells. The method reveals density, diffusion coefficient, and activation energy of mobile ions within a single temperature sweep and offers an intuitive way to distinguish mobile ion species. We apply the TAIC technique to quantify mobile ions of MAPbI3 and triple-cation perovskite solar cells. We find a higher activation energy and a lower diffusion coefficient in the triple-cation devices. TAIC measurements are a simple yet powerful tool to better understand ion migration in perovskite solar cells.

cond-mat.mtrl-sci↗

arXiv2025

Floquet-Engineering of Feshbach Resonances in Ultracold Gases

Scattering resonances are fundamental in science, spanning energy scales from stellar nuclear fusion to ultracold collisions. In ultracold quantum gases, magnetic Feshbach resonances have transformed quantum many-body research by enabling precise interaction control between atoms. Here, we demonstrate unprecedented control to engineer new Feshbach resonances at tunable positions via Floquet driving in a $^{6}$Li atom gas, achieved through strong magnetic field modulation at MHz frequencies. This periodic modulation creates new resonances whenever dressed molecular levels cross the atomic threshold. By adding a second modulation at twice the base frequency, we tune the asymmetry of resonance loss profiles and suppress two-body losses from Floquet heating. This technique enhances control over atomic interactions, expanding possibilities for quantum simulations of complex systems and studies of exotic quantum phases.

cond-mat.quant-gas↗

arXiv2024

Versatile Top-Down Patterning Technique for Perovskite On-Chip Integration

Metal-halide perovskites (MHPs) have exciting optoelectronic properties and are under investigation for various applications, such as photovoltaics, light-emitting diodes, and lasers. An essential step towards exploiting the full potential of this class of materials is their large-scale, on-chip integration with high-resolution, top-down patterning. The development of such patterning methods for perovskite films is challenging because of their intrinsic ionic nature and adverse reactions with the solvents used in standard lithography processes. Here, we introduce a versatile and precise method comprising photolithography and reactive ion etching (RIE) processes that can be tuned to accommodate different perovskite compositions and morphologies. Our method utilizes conventional photoresists at reduced temperatures to create micron-sized features down to 1 ${\mu}$m, providing high reproducibility from chip to chip. The patterning technique is validated through atomic force microscopy (AFM), X-ray diffraction (XRD), optical spectroscopy, and scanning electron microscopy (SEM). It enables the scalable and high-throughput on-chip monolithic integration of MHPs.

cond-mat.mtrl-sci↗

arXiv2023

Unravelling Interaction and Temperature Contributions in Unpolarized Trapped Fermionic Atoms in the BCS Regime

In the BCS limit density profiles for unpolarized trapped fermionic clouds of atoms are largely featureless. Therefore, it is a delicate task to analyze them in order to quantify their respective interaction and temperature contributions. Temperature measurements have so far been mostly considered in an indirect way, where one sweeps isentropically from the BCS to the BEC limit. Instead we suggest here a direct thermometry, which relies on measuring the column density and comparing the obtained data with a Hartree-Bogoliubov mean-field theory combined with a local density approximation. In case of an attractive interaction between two-components of $^{6}$Li atoms trapped in a tri-axial harmonic confinement we show that minimizing the error within such an experiment-theory collaboration turns out to be a reasonable criterion for analyzing in detail measured densities and, thus, for ultimately determining the sample temperatures. The findings are discussed in view of various possible sources of errors.

cond-mat.quant-gas↗

arXiv2023

Stabilizing an ultracold Fermi gas against Fermi acceleration to superdiffusion through localization

Anderson localization, i.e., destructive quantum interference of multiple-scattering paths, halts transport entirely. Contrarily, time-dependent random forces expedite transport via Fermi acceleration, proposed as a mechanism for high-energy cosmic rays. Their competition creates interesting dynamics, but experimental observations are scarce. Here, we experimentally study the expansion of an ultracold Fermi gas inside time-dependent disorder and observe distinct regimes from sub- to superdiffusion. Unexpectedly, quantum interference counteracts acceleration in strong disorder. Our system enables the investigation of Fermi acceleration in the quantum-transport regime.

cond-mat.quant-gas↗

arXiv2023

What can we learn from diffusion about Anderson localization of a degenerate Fermi gas?

Disorder can fundamentally modify the transport properties of a system. A striking example is Anderson localization, suppressing transport due to destructive interference of propagation paths. In inhomogeneous many-body systems, not all particles are localized for finite-strength disorder, and the system can become partially diffusive. Unravelling the intricate signatures of localization from such observed diffusion is a long-standing problem. Here, we experimentally study a degenerate, spin-polarized Fermi gas in a disorder potential formed by an optical speckle pattern. We record the diffusion in the disordered potential upon release from an external confining potential. We compare different methods to analyze the resulting density distributions, including a new method to capture particle dynamics by evaluating absorption-image statistics. Using standard observables, such as diffusion exponent and coefficient, localized fraction, or localization length, we find that some show signatures for a transition to localization above a critical disorder strength, while others show a smooth crossover to a modified diffusion regime. In laterally displaced disorder, we spatially resolve different transport regimes simultaneously which allows us to extract the subdiffusion exponent expected for weak localization. Our work emphasizes that the transition toward localization can be investigated by closely analyzing the system's diffusion, offering ways of revealing localization effects beyond the signature of exponentially decaying density distribution.

cond-mat.quant-gas↗

arXiv2023

Stability and sensitivity of interacting fermionic superfluids to quenched disorder

The microscopic pair structure of superfluids has profound consequences on their properties. Delocalized pairs are predicted to be less affected by static disorder than localized pairs. Ultracold gases allow tuning the pair size via interactions, where for resonant interaction superfluids shows largest critical velocity, i.e. stability against perturbations. The sensitivity of such fluids to strong, time-dependent disorder is less explored. Here, we investigate ultracold, interacting Fermi gases across various interaction regimes after rapid switching optical disorder potentials. We record the ability for quantum hydrodynamic expansion of the gas to quantify its long-range phase coherence. Contrary to static expectations, the Bose-Einstein condensate (BEC) exhibits significant resilience against disorder quenches, while the resonantly interacting Fermi gas permanently loses quantum hydrodynamics. Our findings suggest an additional absorption channel perturbing the resonantly interacting gas as pairs can be directly affected by the disorder quench.

cond-mat.quant-gas↗

arXiv2022

Understanding and Minimizing $V_{OC}$ Losses in All-Perovskite Tandem Photovoltaics

All-perovskite tandem solar cells promise high photovoltaic performance at low cost. So far however, their efficiencies cannot compete with traditional inorganic multi-junction solar cells and they generally underperform in comparison to what is expected from the isolated single junction devices. Understanding performance losses in all-perovskite tandem solar cells is a crucial aspect that will accelerate advancement. Here, we perform extensive selective characterization of the individual sub-cells to disentangle the different losses and limiting factors in these tandem devices. We find that non-radiative losses in the high-gap subcell dominate the overall recombination losses in our baseline system as well as in the majority of literature reports. We consecutively improve the high-gap perovskite subcell through a multi-faceted approach, allowing us to enhance the open-circuit voltage ($V_{OC}$) of the subcell by up to 120 mV. Due to the (quasi) lossless indium oxide interconnect which we employ for the first time in all-perovskite tandems, the $V_{OC}$ improvements achieved in the high-gap perovskites translate directly to improved all-perovskite tandem solar cells with a champion $V_{OC}$ of 2.00 V and a stabilized efficiency of 23.7%. The efficiency potential of our optimized all-perovskite tandems reaches 25.2% and 27.0% when determined from electro- and photo-luminescence respectively, indicating significant transport losses as well as imperfect energy-alignment between the perovskite and the transport layers in the experimental devices. Further improvements to 28.4% are possible considering the bulk quality of both absorbers measured using photo-luminescence on isolated perovskite layers. Our insights therefore not only show an optimization example but a generalizable evidence-based strategy for optimization utilizing optical sub-cell characterization.

cond-mat.mtrl-sci↗

arXiv2022

Revealing the doping density in perovskite solar cells and its impact on device performance

Traditional inorganic semiconductors can be electronically doped with high precision. Conversely, there is still conjecture regarding the assessment of the electronic doping density in metal-halide perovskites, not to mention of a control thereof. This paper presents a multifaceted approach to determine the electronic doping density for a range of different lead-halide perovskite systems. Optical and electrical characterisation techniques comprising intensity-dependent and transient photoluminescence, AC Hall effect, transfer-length-methods, and charge extraction measurements were instrumental in quantifying an upper limit for the doping density. The obtained values are subsequently compared to the charge on the electrodes per unit volume at short-circuit conditions, which amounts to roughly $10^{16}$ cm$^{-3}$. This figure equals the product of the capacitance $C$ and the built-in potential $V_\mathrm{bi}$ and represents the critical limit below which doping-induced charges do not influence the device performance. The experimental results demonstrate consistently that the doping density is below this critical threshold ($<10^{12}$ cm$^{-3}$ which means $<CV_\mathrm{bi}$ per unit volume) for all common lead-based metal-halide perovskites. Nevertheless, although the density of doping-induced charges is too low to redistribute the built-in voltage in the perovskite active layer, mobile ions are present in sufficient quantities to create space-charge-regions in the active layer, reminiscent of doped pn-junctions. These results are well supported by drift-diffusion simulations which confirm that the device performance is not affected by such low doping densities.

cond-mat.mtrl-sci↗

arXiv2020

Relaxed current matching requirements in highly luminescent perovskite tandem solar cells and their fundamental efficiency limits

Here we use time-resolved and steady-state optical spectroscopy on state-of-the-art low- and high-bandgap perovskite films for tandems to quantify intrinsic recombination rates and absorption coefficients. We apply these data to calculate the limiting efficiency of perovskite-silicon and all-perovskite two-terminal tandems employing currently available bandgap materials as 42.0 % and 40.8 % respectively. By including luminescence coupling between sub-cells, i.e. the re-emission of photons from the high-bandgap sub-cell and their absorption in the low-bandgap sub-cell, we reveal the stringent need for current matching is relaxed when the high-bandgap sub-cell is a luminescent perovskite compared to calculations that do not consider luminescence coupling. We show luminescence coupling becomes important in all-perovskite tandems when charge carrier trapping rates are < 10$^{6}$ s$^{-1}$ (corresponding to carrier lifetimes longer than 1 ${\mu}$s at low excitation densities) in the high-bandgap sub-cell, which is lowered to 10$^{5}$ s$^{-1}$ in the better-bandgap-matched perovskite-silicon cells. We demonstrate luminescence coupling endows greater flexibility in both sub-cell thicknesses, increased tolerance to different spectral conditions and a reduction in the total thickness of light absorbing layers. To maximally exploit luminescence coupling we reveal a key design rule for luminescent perovskite-based tandems: the high-bandgap sub-cell should always have the higher short-circuit current. Importantly, this can be achieved by reducing the bandgap or increasing the thickness in the high-bandgap sub-cell with minimal reduction in efficiency, thus allowing for wider, unstable bandgap compositions (>1.7 eV) to be avoided. Finally, we experimentally visualise luminescence coupling in an all-perovskite tandem device stack through cross-section luminescence images.

cond-mat.mtrl-sci↗