Browse Articles

Coherent control of the efficient ladder-type population transfer by four-color harmonic laser pulses

The ladder-type population transfer of the HF molecule steered by four-color harmonic laser pulses (HLPs) is investigated using the time-dependent quantum wave packet method. It is found that although there exist large background excitations and many (resonant) transition pathways during the driving, nearly 100% of the population could be transferred to the target state. In particular, such a process could be coherently controlled by changing the phases of the four HLPs, especially the phases of the fundamental and second HLPs, which can be accounted for in large part by the combined effects of the corresponding transition pathways and the maximal amplitude of the total electric field. However, for manipulating the phases of the third and fourth HLPs, both the changes in the maximal electric field amplitude and the asymmetry size fail to guide the variation of the target-state population because of the correlated effects of all these transition pathways, particularly the ones that do not contain the third and fourth HLP excitations. Importantly, our results also denote that the variation of the maximal electric field amplitude may give a well qualitative prediction about the phase-controlled population when the manipulated phase is directly related to all the transition pathways, which is the general case in the widely used two-color scheme. In addition, the maximal amplitude of the total electric field tends to play a more important role than its asymmetry size in the phase-controlled population transfer process.

Characterization of chromium impurities in <i>β</i>-Ga2O3

Chromium is a common transition-metal impurity that is easily incorporated during crystal growth. It is perhaps best known for giving rise to the 694.3 nm (1.786 eV) emission in Cr-doped Al2O3, exploited in ruby lasers. Chromium has also been found in monoclinic gallium oxide, a wide-bandgap semiconductor being pursued for power electronics. In this work, we thoroughly characterize the behavior of Cr in Ga2O3 through theoretical and experimental techniques. β-Ga2O3 samples are grown with the floating zone method and show evidence of a sharp photoluminescence signal, reminiscent of ruby. We calculate the energetics of formation of Cr from first principles, demonstrating that Cr preferentially incorporates as a neutral impurity on the octahedral site. Cr possesses a quartet ground-state spin and has an internal transition with a zero-phonon line near 1.8 eV. By comparing the calculated and experimentally measured luminescence lineshape function, we elucidate the role of coupling to phonons and uncover features beyond the Franck–Condon approximation. The combination of strong emission with a small Huang–Rhys factor of 0.05 and a technologically relevant host material renders Cr in Ga2O3 attractive as a quantum defect.

Controlling bulk electrostatics in electrolytes by surface polarization

The benchmark theory of hydration forces that relies on the phenomenological expressions developed by Marčelja and Radić (MR) has recently been revived by experimental, computational, and theoretical advances. Here, we consider the effect of surface polarization on electrolytes in a slab geometry by combining the MR approach to polarization with Poisson–Boltzmann theory. Due to the coupling of bulk and surface fields, not only is the electrostatics modified by polarization, but maybe even more importantly, vice versa: a finite polarization at the wall is sufficient to generate a finite electrostatic potential even in the absence of net charges on the wall. We determine the polarization and electrostatic potential profiles and the free energy of the system. Our results show that the presence of surface polarization alone suffices to imprint the bulk structural properties on the electrostatic field in an electrolyte.

Ruddlesden–Popper and perovskite phases as a material platform for altermagnetism

The subclass collinear antiferromagnets that break spin Kramers degeneracy—thereby exhibiting ferromagnet-like properties—offer exciting opportunities in magnetism, which motivates the expansion of the material base for these so-called altermagnets. Here, we demonstrate that Ruddlesden–Popper and perovskite phases offer a rich material platform for altermagnetic behavior. Using first-principles calculations, we demonstrate altermagnetism in prototypical nickel-based compounds such as La2NiO4 and identify additional candidates, including the superconducting La3Ni3O7 and the multiferroic BiFeO3. These materials span insulating, semiconducting, and metallic conduction types, with computed nonrelativistic spin splittings reaching up to 250 meV. Our analysis further reveals the presence of accidental nodes and distinct spin-momentum texture topologies at the Brillouin-zone boundary, suggesting a refined classification beyond the initial d-wave and higher even-parity wave classes. Additionally, we address formal inconsistencies in the traditional classification of magnetically ordered systems, proposing resolutions within the altermagnetic framework. Finally, we highlight the potential for altermagnetic behavior of ferrimagnets and weak ferromagnets, broadening the scope for future exploration.

Assessing the performance of coupled-trajectory schemes on full-dimensional two-state linear vibronic coupling models

We investigate the performance of coupled-trajectory methods for nonadiabatic molecular dynamics in simulating the photodynamics of 4-(dimethylamino)benzonitrile (DMABN) and fulvene, with electronic structure provided by linear vibrational coupling models. We focus on the coupled-trajectory mixed quantum-classical (CTMQC) algorithm and on the (combined) coupled-trajectory Tully surface hopping [(C)CTTSH] in comparison to independent-trajectory approaches, such as multi-trajectory Ehrenfest and Tully surface hopping. Our analysis includes not only electronic populations but also additional electronic and nuclear properties in position and momentum space. For both DMABN and fulvene, the recently developed CCTTSH algorithm successfully resolves the internal inconsistencies of coupled-trajectory Tully surface hopping. Instead, we find that DMABN highlights a significant weakness of CTMQC, which arises when the trajectories remain for a long time in the vicinity of a region of strong nonadiabaticity.

Tunable anisotropic surface Tamm modes in one-dimensional phosphorene–dielectric photonic crystals

In this work, surface Tamm modes have been demonstrated to exist between a seminfinite 1D phosphorene-based crystal and a homogeneous dielectric material. We computed dispersion relations for surface Tamm modes propagating along one of the two principal directions in phosphorene (zig-zag and armchair directions). Surface Tamm mode decaying lengths are obtained for different charge carrier densities, which can be externally tuned via gate-voltage techniques in phosphorene. We have shown that these modes are plasmonic in nature, lying below light lines of external and interlayer dielectric media. Considering the conductivity model for phosphorene at low THz regime, we have also shown the existence of plasmonic bands for phosphorene-based crystals, showing that plasmonic mode coupling is possible for the THz frequency regimes.

Numerically stable resonating Hartree–Fock

The simulation of excited states at low computational cost remains an open challenge for electronic structure (ES) methods. While much attention has been given to orthogonal ES methods, relatively little work has been done to develop nonorthogonal ES methods for excited states, particularly those involving nonorthogonal orbital optimization. We present here a numerically stable formulation of the Resonating Hartree–Fock (ResHF) method that uses the matrix adjugate to remove numerical instabilities arising from nearly orthogonal orbitals, and as a result, we demonstrate improvements to ResHF wavefunction optimization. We then benchmark the performance of ResHF against complete active space self-consistent field in the avoided crossing of LiF, the torsional rotation of ethene, and the singlet–triplet energy gaps of a selection of small molecules. ResHF is a promising excited state method because it incorporates the orbital relaxation of state-specific methods, while retaining the correct state crossings of state-averaged approaches. Our open-source ResHF implementation, yucca, is available on GitLab.

Direct measurement of lattice behavior during femtosecond laser-driven shock front formation in copper

Femtosecond laser-driven shock waves exhibit characteristic features that form distinctive microstructures not formed by plate impacts or nanosecond laser-driven shock waves. A key to understanding this phenomenon is understanding the lattice behavior inside the shock front, which is the boundary between the ambient and shock compression states. However, direct measurements of the lattice spacing inside a femtosecond laser-driven shock front have not yet been performed. Here, we report in situ measurements of lattice spacing using x-ray free electron laser diffraction with a pulse width of &amp;lt;10 fs during the shock rise in single-crystal copper irradiated directly in air with a femtosecond laser pulse on the order of 1014 W/cm2 at a pulse width of 101 fs. The lattice spacing of the femtosecond laser-irradiated single-crystal Cu (002) plane starts to compress 6.3 ps after femtosecond laser irradiation. It takes 15.7 ps for the plane to reach peak compression, at which point the compressive elastic strain is 24.3%. Therefore, the shock front was found to form at an elastic compressive strain rate of 1.55 × 1010/s in this shock-driving situation. It is suggested that the initiation of plasticity under such ultrafast deformation at the most elastic compression is based on both dislocation multiplication and dislocation generation mechanisms.

Stereodynamics of cold HD and D2 collisions with He

We present a comprehensive quantum mechanical study of stereodynamic control of HD + He and D2 + He collisions that have been probed experimentally by Perreault et al. [J. Phys. Chem. Lett. 13, 10912 (2022)] using Stark-induced adiabatic Raman passage (SARP) techniques. Our calculations utilize a highly accurate full-dimensional H2 + He interaction potential with diagonal Born–Oppenheimer correction appropriate for HD and D2 isotopomers. The results show that rotational quenching of HD from j = 2 → j′ = 0 in v = 2, j = 2 → j′ = 1 in v = 2 and v = 4, and j = 4 → j′ = 3 in v = 4 is dominated by an l = 1 shape resonance located between 0.1 and 1.0 cm−1. For collision energies less than 0.1 cm−1, isotropic scattering prevails. An l = 1 resonance centered around 0.02 cm−1 is also found to dominate the j = 2 → j′ = 0 and j = 4 → j′ = 2 transitions in v = 4 for He–D2 collisions consistent with our prior studies of Δj = −2 transition in He + D2(v = 2, j = 2) collisions. Our analysis does not support the hypothesis of Perreault et al. [J. Phys. Chem. Lett. 13, 10912 (2022)] that a strong l = 2 resonance controls the angular distribution for Δj = −2 transition for both systems. Despite improvements in the development of the potential energy surface, a good agreement with SARP experiments for v = 2 is achieved only when contributions from collision energies less than 1.0 cm−1 were excluded in the computation of velocity averaged differential rate coefficients for both systems. This could be due to some uncertainties in the velocity spread in the experiment that employs co-propagation of the collision partners and possibly, the neglect of transverse velocities in the simulation of the experiment.

Effect of micro-alloying solute addition on the stacking fault energy of Al–Cu alloys: A computational and experimental approach

Present study reports the individual and synergistic effect of Mn and Zr solute addition to the stacking fault energy (SFE) of Al–Cu alloys through first principle-based density functional theory (DFT) calculations and experimental x-ray diffraction line profile analysis (XRDLPA). DFT calculations suggest that while Mn addition enhances SFE for Al–Cu–Mn alloys, Zr addition significantly lowers it for Al–Cu–Zr alloys with their co-addition in Al–Cu–Mn–Zr alloys producing an intermediate SFE. XRDLPA of these alloys in cold-rolled conditions validates the trend obtained from the theoretical calculations with consistency and absolute values to a satisfactory extent. Uniaxial tensile testing finally confirms SFE variation from the mechanical properties, especially the ductility of the cold-rolled alloys, which corroborates well with the variation in their strain hardening response as well.

Hamiltonian replica-exchange method α-REMD for ring spearing elimination in polymers

Accurate prediction of polymer properties using molecular dynamics (MD) simulations requires a properly relaxed starting structure. Polymer models built from scratch by specialized algorithms (self-avoiding random walk, Monte Carlo, etc.) are far from relaxed and, moreover, often possess a large number of structural defects: close contacts between atoms, wrong bond distances, voids, unfavorable molecular conformations or packing, etc. This is especially problematic for ring-containing polymers whose initial structures also include ring spearing (bonds passing through cycles, including benzene rings). All these defects must be eliminated before running an MD simulation to correctly predict polymer properties. Short MD simulations can be enough to remove close contacts; however, ring spearing elimination and general structure relaxation cannot be achieved this way. In this work, we propose α-Replica Exchange MD (α-REMD)–a Hamiltonian replica-exchange MD protocol that reliably eliminates ring spearing defects and performs a general relaxation of the system. Its efficiency is demonstrated on five polyethersulfones whose initial geometries contained numerous ring intersections that were completely removed by α-REMD.

A machine learning approach to predicting the spall strength of metals and alloys

Spall strength is a critical property that characterizes a material's resistance to dynamic failure under impact or shock compression loading. Accurate prediction and control of spall strength are essential for designing materials used in armors, vehicle components, structural barriers, and mining. Traditional methods for measuring the spall strength are slow, destructive, and expensive, while existing models offer limited predictive accuracy. This study introduces a machine learning regression model that rapidly and accurately predicts the spall strength of metals and alloys at strain rates in the range of 105–106 s−1. Trained on a dataset of over 70 metals and alloys and validated with recent data, the model identifies tensile yield strength and bulk modulus as key factors, with higher values of these properties correlating with increased spall strength. The findings offer an efficient method for screening metals and alloys and guiding the selection of high-spall-strength candidates for targeted experimental validation.

Noise-induced synchronization in coupled quantum oscillators

We consider the quantum dynamics of a pair of coupled quantum oscillators coupled to a common correlated dissipative environment. The resulting equations of motion for both the operator moments and covariances can be integrated analytically using the Lyapunov equations. We find that for fully correlated and fully anti-correlated environments, the oscillators relax into a phase-synchronized state that persists for long-times when the two oscillators are nearly resonant and (essentially) forever if the two oscillators are in resonance. We identify an exceptional point that indicates the onset of broken symmetry between an unsynchronized and synchronized dynamical phase of the system as correlations within the environment are increased. We also show that the environmental noise correlation leads to quantum entanglement, and all the correlations between the two oscillators are purely quantum mechanical in origin. This work provides a robust mathematical foundation for understanding how long-lived exciton coherences can be linked to vibronic correlation effects.

Retraction: “Field induced phase transition and magnetic phase diagram of multiferroic Ca2CoSi2O7” [J. Appl. Phys. <b>133</b>, 215903 (2023)]

Dynamic heterogeneity of short semi-crystalline polymer chains during recrystallization

The instant crystallization of semi-crystalline polymers has become possible following the recent advances in Fast Scanning Calorimetry (FSC) and enables us to make a bridge between the time scale available experimentally with those accessible with computer simulations. Although the FSC observations have provided new information on the crystallization kinetics and evolution of the crystals, the molecular details on the chain exchange events between the ordered and disordered domains of crystals have remained elusive. Using molecular dynamics simulations, we examined the detailed chain dynamics and thermodynamics of polyamide 6 (PA6) system under two heating treatments: (i) quenching PA6 melt deeply below the melting temperature Tm and (ii) annealing the resulting quenched system to a temperature close to Tm. We categorized the chains into mobile amorphous fraction (MAF) and rigid amorphous fraction (RAF), based on the length of consecutive chain’s bond angles in the trans state. In the deep quenched system close to the glass transition temperature Tg, the mobility of the MAF chains is strongly suppressed and they remain in the glassy state. However, upon rising the temperature close to melting temperature, the system undergoes recrystallization, leading to the coexistence of RAF and supercooled liquid MAF chains. The highly mobile unentangled MAF chains explore the interphase domains, and during the late-stage of crystallization, they are thermally translocated into the lamella by reducing the fold number of RAF chains. The chain mobility in the annealed system could potentially lead to improved biodegradation in semi-crystalline chains.

Frequency locking: A distinctive feature of the coherent population trapping

We investigate the use of phase modulation to generate an error signal for frequency stabilization to a reference atomic transition. The study focuses on the high-frequency modulation regime. Our theoretical analysis shows that, in the case of coherent population trapping, the maximal slope of the error signal remains constant as the modulation frequency increases, whereas it steadily decreases for other types of resonances. Experimental results with 87Rb atoms confirm this behavior under comparison with the double radio-optical resonance.

Electrostatic interactions in nucleosome and higher-order structures are regulated by protonation state of histone ionizable residue

The nucleosome serves as the fundamental unit of chromatin organization, with electrostatic interactions acting as the driving forces in the folding of nucleosomes into chromatin. Perturbations around physiological pH conditions can lead to changes in the protonation states of titratable histone residues, impacting nucleosome surface electrostatic potentials and interactions. However, the effects of proton uptake or release of histone ionizable groups on nucleosome–partner protein interactions and higher-order chromatin structures remain largely unexplored. Here, we conducted comprehensive analyses of histone titratable residue pKa values in various nucleosome contexts, utilizing 96 experimentally determined complex structures. We revealed that pH-induced changes in histone residue protonation states modulated nucleosome surface electrostatic potentials and significantly influenced nucleosome–partner protein interactions. Furthermore, we observed that proton uptake or release often accompanied nucleosome–partner protein interactions, facilitating their binding processes. In addition, our findings suggest that alterations in histone protonation can also regulate nucleosome self-association, thereby modulating the organization and dynamics of higher-order chromatin structure. This study advances our understanding of nucleosome–chromatin factor interactions and how chromatin organization is regulated at the molecular level.

Global model for flashover phenomena in vacuum: A comprehensive perspective

Flashover is an electrical breakdown along the dielectric-gas/vacuum/liquid interface under high electric field excitation. Surface flashover phenomena in vacuum greatly impede a variety of vacuum insulation devices and systems. Here, a comprehensive perspective of the vacuum flashover global model is provided to integrate existing understandings and highlight featured prospects of the flashover mechanisms, mitigation approaches, and applications. An overview of physical processes involved in the entire vacuum flashover process is first given. Recent advances and perspectives for the understanding of these processes are then discussed separately, including the surface discharge above dielectric, and the charge transport and breakdown within dielectric bulk and surface layer. Scaling laws and empirical formulas for flashover threshold prediction are assessed as well. The mechanisms of recent vacuum flashover mitigation approaches are analyzed, such as using physical structures and geometrical modifications, material-based approaches, and applying external electromagnetic field, and possible novel flashover mitigation methods are predicted. In addition, potential applications using vacuum flashover are discussed. Finally, promising research topics, imminent challenges, and open questions of the vacuum flashover studies are presented. It might be instructive for the fundamental and application research studies of surface flashover in vacuum in future.

Learning transition path and membrane topological signatures in the folding pathway of bacteriorhodopsin (BR) fragment with artificial intelligence

Membrane protein folding in the viscous microenvironment of a lipid bilayer is an inherently slow process that challenges experiments and computational efforts alike. The folding kinetics is moreover associated with topological modulations of the biological milieu. Studying such structural changes in membrane-embedded proteins and understanding the associated topological signatures in membrane leaflets, therefore, remain relatively unexplored. Herein, we first aim to estimate the free energy barrier and the minimum free energy path (MFEP) connecting the membrane-embedded fully and partially inserted states of the bacteriorhodopsin fragment. To achieve this, we have considered independent sets of simulations from membrane-mimicking and membrane-embedded environments, respectively. An autoencoder model is used to elicit state-distinguishable collective variables for the system utilizing membrane-mimicking simulations. Our in-house Expectation Maximized Molecular Dynamics algorithm is initially used to deduce the barrier height between the two membrane-embedded states. Next, we develop the Geometry Optimized Local Direction search as a post-processing algorithm to identify the MFEP and the corresponding peptide conformations from the autoencoder-projected trajectories. Finally, we apply a graph attention neural network (GAT) model to learn the membrane surface topology as a function of the associated peptide structure, supervised by the membrane-embedded simulations. The resultant GAT model is then utilized to predict the membrane leaflet topology for the peptide structures along MFEP, obtained from membrane-mimicking simulations. The combined framework is expected to be useful in capturing key phenomena accompanying folding transitions in membranes. We discuss opportunities and avenues for further development.

Cryogenic magnetocaloric effect of rare-earth halide perovskites CsEuCl3 and CsEuBr3

Adiabatic demagnetization refrigeration (ADR) offers a compelling alternative to traditional ultra-low temperature technologies by eliminating the reliance on scarce 3He and gravitational constraints. The key of ADR development is to find magnetic refrigerants with exceptional magnetocaloric properties. In this study, polycrystalline CsEuCl3 and CsEuBr3 halide perovskites were synthesized by a simple solid-phase reaction method. CsEuCl3 crystallizes in a tetragonal structure, while CsEuBr3 forms an orthorhombic structure, both exhibiting distorted perovskite structures. CsEuCl3 undergoes an antiferromagnetic ordering at TN ∼ 1.1 K, whereas CsEuBr3 shows no magnetic ordering above 0.4 K. Curie–Weiss fitting analysis reveals antiferromagnetic interactions in both compounds, with CsEuBr3 displaying stronger antiferromagnetic coupling. The maximum magnetic entropy change (−ΔSM) values under magnetic field changes of 0–2 T and 0–5 T are 22.4 and 38.6 J kg−1 K−1 at 1.4 K for CsEuCl3, and 15.7 and 26.4 J kg−1 K−1 at 0.4 K for CsEuBr3, respectively. These findings underscore the potential of CsEuCl3 and CsEuBr3 as promising candidates for cryogenic magnetic refrigeration applications.