Weak Impurity Research Articles

Event-Based Weak Oil Impurity Detection

Event-Based Weak Oil Impurity Detection

Observation of nontrivial Zak phase induced topological states in glow discharge plasma

Plasma blackout, which contains ablative impurities, strongly attenuates the signal of the reentry spacecraft. Traditional methods focus on mitigating electron densities and impurities around the antenna, and metamaterial-based electromagnetic methods have yet to be proven experimentally. We simulate the plasma blackout problem using laboratory plasma supported by gas discharge technology. Alumina pillars are embedded in the plasma background to form plasma photonic crystals, while topological phase transitions are achieved by shrinking and expanding pillars within a unit cell. The topological edge states (TESs) that are insensitive to weak impurities in the transport path are verified theoretically and experimentally. We introduce the glide-reflection (GR) symmetry in the nontrivial lattices to obtain the gapless edge states, which are exclusively observed in the acoustic systems. Meanwhile, the Δω of the gapless TES increases with the electron densities, ensuring a wide communication bandwidth. Furthermore, the strong coupling of heterostructure with GR symmetry in plasma photonic crystals is elucidated. Our work not only provides a new approach to the blackout communication problem but can also serve as a nascent experimental platform to investigate topological electromagnetic phenomena.

Simplification of pharmacopoeial liquid chromatography methods for related substances of statins by hyphenated ultraviolet and charged aerosol detection

For a more comprehensive characterization of a drug substance and its impurities, multidetector approaches are a helpful tool in liquid chromatography. In particular, the relatively inexpensive hyphenation of the ultraviolet (UV) with the charged aerosol detector (CAD) extends the scope from UV-active to non- or weak chromophore analytes, respectively. In this study, the chromatographic methods of the test for related substances of simvastatin and lovastatin in the European Pharmacopoeia were adapted to UV-CAD and thus allowing a more sophisticated detection of the weak chromophore dihydro impurity besides the other UV-active impurities. The compendial gradient program for simvastatin had to be modified (lowered initial acetonitrile percentage and increased gradient slope) because an additional critical peak pair emerged with the Hypersil C18 BDS column used here. Therefore, a Plackett-Burman design with 11 factors (including 4 dummy factors) was chosen to evaluate robustness of the adapted method. The flow rate, initial acetonitrile percentage, and column temperature were identified as three critical parameters that had to be carefully observed. Finally, the validity of the method for simultaneous detection of dihydrosimvastatin with CAD and of lovastatin and simvastatin as examples of UV detection was verified according to ICH Q2 (R1). In the case of lovastatin, the direct comparison with the pharmacopoeial method reveal that a determination with CAD is the more sensitive method.

Quasiparticle Interference as a Direct Experimental Probe of Bulk Odd-Frequency Superconducting Pairing.

We show that quasiparticle interference (QPI) due to omnipresent weak impurities and probed by Fourier transform scanning tunneling microscopy and spectroscopy acts as a direct experimental probe of bulk odd-frequency superconducting pairing. Taking the example of a conventional s-wave superconductor under applied magnetic field, we show that the nature of the QPI peaks can only be characterized by including the odd-frequency pairing correlations generated in this system. In particular, we identify that the defining feature of odd-frequency pairing gives rise to a bias asymmetry in the QPI, present generically in materials with odd-frequency pairing irrespective of its origin.

Quantum fidelity of the Aubry-André model and the exponential orthogonality catastrophe

Advances in experimental tools allow to study quantum fidelity in unprecedented controlled settings. While the fidelity in metals, when adding a local impurity, is well known to show the Anderson orthogonality catastrophe (AOC), there remain outstanding questions in other quantum phases and settings. Here, we aim to tackle these by exploring systematically the ground-state fidelity of the Fermi liquid in the (extended) Aubry-Andr\'e (AA) model, which allows to explore the AOC in both localized extended and critical phases. We discover that the AOC is typically exponential in the critical regime of the AA model and at the mobility edge of the extended AA model for an extended impurity, while it decays in the AA model with a power law for a weak single-site impurity. We explain this in terms of critical correlations and multipoint correlations. The OC is found to be exponential in the insulating regime, due to a fundamentally different, statistical mechanism, which is explained in detail. Furthermore, we consider a parametric perturbation to the AA model, and find an exponential OC numerically, in agreement with an analytical derivation which we provide here.

Revealing sign-reversal s+− -wave pairing by quasiparticle interference in the heavy-fermion superconductor CeCu2Si2

Recent observations of two nodeless gaps in superconducting CeCu$_2$Si$_2$ have raised intensive debates as to its exact gap structure of either sign-reversal ($s^{+-}$) or sign-preserving ($s^{++}$) pairing. Here we investigate the quasiparticle interference (QPI) using realistic Fermi surface topology for both weak and strong interband impurity scatterings. Our calculations of the QPI and integrated antisymmetrized local density of states reveal qualitative distinctions between $s^{+-}$ and $s^{++}$ pairing states, which include the intragap impurity resonance and a significant energy-dependence difference between two gap energies. Our predictions provide a guide for phase-sensitive QPI measurements to uncover decisively the true pairing symmetry in the heavy-fermion superconductor CeCu$_2$Si$_2$.

Isolation and Purification of a Hydrophobic Non-Ribosomal Peptide from an Escherichia coli Fermentation Broth

Non-ribosomal peptide synthases (NRPSs) generate versatile bioactive peptides by incorporating non-proteinogenic amino acids and catalyzing diverse modifications. Here, we developed an efficient downstream process for the capture, intermediate purification and polishing of a rhabdopeptide (RXP) produced by the NRPS VietABC. Many typical unit operations were unsuitable due to the similar physical and chemical properties of the RXP and related byproducts. However, we were able to capture the RXP from a fermentation broth using a hydrophobic resin (XAD-16N), resulting in a 14-fold increase in concentration while removing salts as well as polar and weak non-polar impurities. We then used ultra-high-performance liquid chromatography (UHPLC) for intermediate purification, with optimized parameters determined using statistical experimental designs, resulting in the complete removal of hydrophobic impurities. Finally, the UHPLC eluents were removed by evaporation. Our three-step downstream process achieved an overall product recovery of 81.7 ± 8.4%.

Effects of disorder in the Fibonacci quasicrystal

We study the properties of the one-dimensional Fibonacci chain, subjected to the placement of on-site impurities. The resulting disruption of quasiperiodicity can be classified in terms of the renormalization path of the site at which the impurity is placed, which greatly reduces the possible amount of disordered behavior that impurities can induce. Moreover, it is found that, to some extent, the addition of multiple, weak impurities can be treated by superposing the individual contributions together and ignoring nonlinear effects. This means that a transition regime between quasiperiodic order and disorder exists, in which some parts of the system still exhibit quasiperiodicity, while other parts start to be characterized by different localisation behaviours of the wavefunctions. This is manifested through a symmetry in the wavefunction amplitude map, expressed in terms of conumbers, and through the inverse participation ratio. For the latter, we find that its average of states can also be grouped in terms of the renormalization path of the site at which the impurity has been placed.

Correlations among STM observables in disordered unconventional superconductors

New developments in scanning tunneling spectroscopy now allow for the spatially resolved measurement of the Josephson critical current ${I}_{c}$ between a tip and a superconducting sample, a nearly direct measurement of the true superconducting order parameter. However, it is unclear how these ${I}_{c}$ measurements are correlated with previous estimates of the spectral gap taken from differential conductance measurements. In particular, recent experiments on an iron-based superconductor found almost no correlation between ${I}_{c}$ and the spectral gap obtained from differential conductance $g=dI/dV$ spectra, reporting instead a more significant correlation between ${I}_{c}$ and the coherence-peak height. Here we point out that the correlation---or the lack thereof---between these various quantities can be naturally explained by the effect of disorder on unconventional superconductivity. Using large-scale numerical simulations of a BCS $d$-wave pair Hamiltonian with many-impurity potentials, we observe that ``substitutional'' disorder models with weak pointlike impurities lead to a situation in which the true superconducting order parameter and ${I}_{c}$ are both uncorrelated with the spectral gap from $dI/dV$ measurements and highly correlated with the coherence-peak heights. The underlying mechanism appears to be the disorder-induced transfer of spectral weight away from the coherence peaks. On the other hand, smooth impurity potentials with a length scale larger than the lattice constant lead to a large positive correlation between the true superconducting order parameter and the spectral gap, in addition to a large correlation between the order parameter and the coherence-peak height. We discuss the applicability of our results to recent Josephson scanning tunneling spectroscopy experiments on iron-based and cuprate high-temperature superconductors.

Low-temperature T -linear resistivity due to umklapp scattering from a critical mode

We consider the transport properties of a model of fermions scattered by a critical bosonic mode. The mode is overdamped and scattering is mainly in the forward direction. Such a mode appears at the quantum critical point for an electronic nematic phase transition and in gauge theories for a U(1) spin liquid. It leads to a short fermion lifetime, violating Landau's criterion for a Fermi liquid. In spite of this, transport can be described by a Boltzmann equation. We include momentum relaxation by umklapp scattering, supplemented by weak impurity scattering. We find that above a very low temperature which scales with ${\mathrm{\ensuremath{\Delta}}}_{q}^{3}$, where ${\mathrm{\ensuremath{\Delta}}}_{q}$ is the minimum umklapp scattering vector, the resistivity is linear in $T$ with a coefficient which is independent of the amount of disorder. This picture holds until the temperature dependent part of the resistivity exceeds that due to impurity scattering. We compare the relaxation time approximation with an exact numerical solution of the Boltzmann equation. Surprisingly we find that, unlike the resistivity, the Hall coefficient strongly deviates from the relaxation time approximation and shows a strong reduction with increasing temperature. We comment on possible comparisons with experiments on high ${T}_{c}$ cuprates.

Thermal Dynamics of Charge Density Wave Pinning in ZrTe_{3}.

Impurity pinning has long been discussed to have a profound effect on the dynamics of an incommensurate charge density wave (CDW), which would otherwise slide through the lattice without resistance. Here, we visualize the impurity pinning evolution of the CDW in ZrTe_{3} using the variable temperature scanning tunneling microscopy. At low temperatures, we observe a quasi-1D incommensurate CDW modulation moderately correlated to the impurity positions, indicating a weak impurity pinning. As we raise the sample temperature, the CDW modulation gets progressively weakened and distorted, while the correlation with the impurities becomes stronger. Above the CDW transition temperature, short-range modulations persist with the phase almost all pinned by impurities. The evolution from weak to strong impurity pinning through the CDW transition can be understood as a result of losing phase rigidity.

Lifetimes of confined optical phonons and the shape of a Raman peak in disordered nanoparticles. II. Numerical treatment

Disorder-induced broadening of optical vibrational eigenmodes in nanoparticles of nonpolar crystals is studied numerically. The methods previously used to treat phonons in defectless particles are adjusted for numerical evaluation of the disordered problem. Imperfections in the form of Gaussian and binary disorders as well as surface irregularities are investigated thoroughly in a wide range of impurity concentrations and disorder strengths. For dilute and weak point-like impurities the regimes of separated and overlapped phonon levels are obtained and the behavior of the linewidth predicted theoretically is confirmed, the crossover scale falls into the actual range of several nanometers. These notions survive for strong dilute impurities, as well. Regimes and crossovers predicted by theory are checked and identified, and minor discrepancies are discussed. To mention a few of them: slower than in theory increasing of the linewidth with the phonon quantum number for weak disorder and only qualitative agreement between theory and numerics for resonant broadening in strong dilute disorder. The novel phenomena discovered numerically are: "mesoscopic smearing" of distribution function in the ensemble of identical disordered particles, inflection of the linewidth dependence on the impurity concentration for light "dense" binary impurities, and position-dependent capability of strong impurity to catch the phonon. It is shown that surface irregularities contribute to the phonon linewidth less than the volume disorder, and their rate reveals faster decay with increasing of the particle size. It is argued that the results of present research are applicable also for quantum dots and short quantum wires.

Lifetimes of confined optical phonons and the shape of a Raman peak in disordered nanoparticles. I. Analytical treatment

Microscopic description of Raman spectra in nanopowders of nonpolar crystals is accomplished by developing the theory of disorder-induced broadening of optical vibrational eigenmodes. Analytical treatment of this problem is performed, and line shape and width are determined as functions of phonon quantum numbers, nanoparticle shape, size, and the strength of disorder. The results are found to be strongly dependent on either the broadened line is separated or it is overlapped with other lines of the spectrum. Three models of disorder, i.e. weak point-like impurities, weak smooth random potential and strong rare impurities are investigated in details. The possibility to form the phonon-impurity bound state is also studied.

Overdoped end of the cuprate phase diagram

Studying the disappearance of superconductivity at the end of the overdoped region of the cuprate phase diagram offers a different approach for investigating the interaction which is responsible for pairing in these materials. In the underdoped region this question is complicated by the presence of charge and stripe ordered phases as well as the pseudogap. In the overdoped region the situation appears simpler with only a normal phase, a superconducting phase and impurity scattering. Here, for the overdoped region, we report the results of a combined dynamic cluster approximation (DCA) and a weak Born impurity scattering calculation for a $t-t'-U$ Hubbard model. We find that a decrease in the $d$-wave pairing strength of the two-particle scattering vertex is closely coupled to changes in the momentum and frequency structure of the magnetic spin fluctuations as the system is overdoped. Treating the impurity scattering within a disordered BCS $d$-wave approximation, we see how the combined effects of the decreasing $d$-wave pairing strength and weak impurity scattering lead to the end of the $T_c$ dome.

Vertically aligned carbon nanotubes, MoS2\u2013rGo based optoelectronic hybrids for NO2 gas sensing

A simple method is developed through drop-casting techniques to assemble a molybdenum disulfide (MoS2)-reduced graphene oxide (rGO) hybrid on vertically aligned carbon nanotubes (VACNTs) to perform as an optoelectronic device for nitrogen dioxide (NO2) gas sensing at room temperature. The VACNT not only forms an ohmic contact with the hybrid material, but also yields a weak charge impurity scattering in the rGo layers across the channel. These features dramatically affect the optical response of the device to the light through which improve the photoresponsivity by up to 236% and the response time by up to 40% compared to the Au contacted device. Next, the fabricated MoS2–rGo/VACNTs device is employed as a resistor gas sensor for NO2 under in situ exposure to the light at room temperature. Under laser illumination, the sensor demonstrates a high sensitivity of ~ 41% at an inlet NO2 concentration of 100 ppm with a complete recovery time of ~ 150 s which shows comparable improvements relative to the sensor performance in dark condition.

Band dispersion of graphene with structural defects

We study the band dispersion of graphene with randomly distributed structural defects using two complementary methods, exact diagonalization of the tight-binding Hamiltonian and implementing a self-consistent T matrix approximation. We identify three distinct types of impurities resulting in qualitatively different spectra in the vicinity of the Dirac point. First, resonant impurities, such as vacancies or 585 defects, lead to stretching of the spectrum at the Dirac point with a finite density of localized states. This type of spectrum has been observed in epitaxial graphene by photoemission spectroscopy and discussed extensively in the literature. Second, nonresonant (weak) impurities, such as paired vacancies or Stone-Wales defects, do not stretch the spectrum but provide a line broadening that increases with energy. Finally, disorder that breaks sublattice symmetry, such as vacancies placed in only one sublattice, open a gap around the Dirac point and create an impurity band in the middle of this gap. We find good agreement between the results of the two methods and also with the experimentally measured spectra.

Pairing instability on a Luttinger surface: A non-Fermi liquid to superconductor transition and its Sachdev-Ye-Kitaev dual

Superconductivity results from an instability of the Fermi surface -- contour of \textit{poles} of the single particle propagator -- to an infinitesimally small attraction between electrons. Here, we instead discuss the analogous problem on a model \textit{Luttinger} surface, or contour of \textit{zeros} of the Green function. At zero temperature ($\beta \rightarrow \infty$) and a critical interaction strength ($u_{c\infty}$) characterized by the residue of self-energy pole, we find that the pair susceptibility diverges leading to a superconducting instability. We evaluate the pair fluctuation partition function and find that the spectral density in the normal state has an interaction-driven, power-law $\frac{1}{\sqrt{\omega}}$ type, van-Hove singularity (vHS) indicating non-Fermi liquid (NFL) physics. Crucially, in the strong coupling limit ($\beta u \gg 1$), the leading order fluctuation free energy terms in the normal state of this NFL-SC transition resemble the equivalent $\left(O(1)\right)$ terms of the Sachdev-Ye-Kitaev (SYK) model. This free energy contribution takes a simple form $-\beta F = \beta u_{c\infty} - \gamma~\text{ln}\left(\beta u_{c \infty}\right)$ where $\gamma$ is a constant equal to $\frac{1}{2}$. Weak impurity scattering ($\tau \gg \beta^{-1}$) leaves the low-energy spectral density unaffected, but leads to an interaction-driven enhancement of superconductivity. Our results shed light on the role played by order-parameter fluctuations in providing the key missing link between Mott physics and strongly coupled toy-models exhibiting gravity duals.

Many-body quantum dynamics and induced correlations of Bose polarons

We study the ground state properties and non-equilibrium dynamics of two spinor bosonic impurities immersed in a one-dimensional bosonic gas upon applying an interspecies interaction quench. For the ground state of two non-interacting impurities we reveal signatures of attractive induced interactions in both cases of attractive or repulsive interspecies interactions, while a weak impurity–impurity repulsion forces the impurities to stay apart. Turning to the quench dynamics we inspect the time-evolution of the contrast unveiling the existence, dynamical deformation and the orthogonality catastrophe of Bose polarons. We find that for an increasing postquench repulsion the impurities reside in a superposition of two distinct two-body configurations while at strong repulsions their corresponding two-body correlation patterns show a spatially delocalized behavior evincing the involvement of higher excited states. For attractive interspecies couplings, the impurities exhibit a tendency to localize at the origin and remarkably for strong attractions they experience a mutual attraction on the two-body level that is imprinted as a density hump on the bosonic bath.

Bosonic Dirac materials on a honeycomb antiferromagnetic Ising model

Motivated by the recent proposal of Bosonic Dirac materials (BDM), we revisited the Ising model on a honeycomb lattice in the presence of the longitudinal and transverse fields. We applied linear spin-wave theory to obtain the magnon dispersion and its Bosonic Dirac points (BDP). At the vicinity of these BDP, the magnon spectrum becomes linear and its crossing bands are robust against a weak impurity, exactly as occur in the fermionic case. We studied the influence of a weak disorder potential over the dispersive edge states and the BDP. Also, using Effective Field Theory, we calculated the quantum and thermal fluctuations over the ground state of the system. The fine-tuning of the fields reveals to control the localization of the BDP. To verify this experimentally, we suggest the compound Ba2CoTeO6.

Low-Temperature Transport in Metals without Inversion Centre

Theory of low temperature kinetic phenomena in metals without inversion center is developed. Kinetic properties of a metal without inversion center are described by four kinetic equations for the diagonal (intra-band) and the off-diagonal (inter-band) elements of matrix distribution function of electrons occupying the states in two bands split by the spin-orbit interaction. The derivation of collision integrals for electron-impurity scatterings and for electron-electron scatterings in a non-centrosymmetric medium is given. Charge, spin and heat transport in the ballistic and the weak impurity scattering regimes is discussed. It is shown that the off-diagonal terms give rise the contribution in charge, spin and heat flows not only due to the interband scattering but also in the collisionless case. The zero-temperature residual resistivity and the residual thermal resistivity are determined by scattering on impurities as well as by the electron-electron scattering.