Session OD01: V: On-Demand Presentations - Available throughout March Meeting

Characterizing mechanical and structural properties of degrading tetra-PEG hydrogels

Controlling degradation of polymer networks is critical for a broad range of applications. Photo-controlled degradation permits spatially-resolved dynamic control of materials properties, in particular mechanical properties of hydrogel-based materials. We use our recently developed dissipative particle dynamics (DPD) framework that captures degradation of hydrogels at the mesoscale to characterize variations in network elasticity as degradation proceeds. Breaking elastically active network strands during degradation leads to a decrease in elastic modulus. We quantify the elastic modulus of degrading gels as a function of the proximity to the reverse gel point. Further, degrading hydrogels are highly heterogeneous systems; hence we quantify the structural and dynamic heterogeneities upon approaching the reverse gel point. Our results allow one to quantify mechanical properties of degrading hydrogel material and could provide guidelines for future design of degrading materials with dynamically controlled mechanical properties.   

Thermal properties of knotted block copolymer rings with charged monomers subjected to short-range interactions

The subject of this talk are the thermal properties of coarse grained knotted copolymer rings fluctuating in a highly screening solution and defined on a simple cubic lattice. The rings contain two kinds of monomers A and B with opposite charges that are subjected to short range interactions. The used model reminds similar models that are applied in proteins.

Similar as in proteins, we find that knotted polymers with an excess of monomers of one kind admit rearrangements leading to metastable states. The temperatures at which the intermediate phase is occurring and other features can be tuned by changing the topology of the system. Finally, a method that is able to capture the relevant properties of the structure of knotted polymer rings in solution will be presented.   

Investigating chemotaxis in asymmetric liposomes

Active systems have gained significant interest across various scientific disciplines. Active motion is an important phenomenon in nature, particularly when related to concentration gradients, a behavior known as chemotaxis. Examples include the directed movement exhibited by bacteria and neutrophils, orchestrated through intricate signaling pathways. In synthetic systems, active and chemotactic behavior has been demonstrated in Janus particles and colloidal structures such as polymersomes and liposomes.

 

This study investigates a novel active system composed of liposomes with encapsulated enzymes. Asymmetry is introduced by incorporating the pore protein alpha-hemolysin, facilitating the diffusion of substrate and products across the vesicle membrane. The resulting asymmetric distribution of species creates a slip velocity on the liposome's surface, resulting in self-propulsion.

 

The nature of the substrate proves to be a critical factor influencing the velocity and direction of the drift of liposomes in a microfluidic channel. Phenomena such as diffusioosmosis and diffusioosmophoresis emerge as inherent components of the motion we measured by tracking polystyrene beads with different surface chemistry in a gradient of urea and glucose. These phenomena are also present in the movement of porated liposomes. In addition to them, a chemotaxis component is observed for the liposomes that have pores. The drift direction and velocity result from all these events and depend on the enzyme/ substrate pair.

 

This research sheds light on some fundamental principles governing liposome chemotaxis with encapsulated enzymes. This system can offer insights into the chemotactic behavior of some natural vesicles and can also be applied in diverse fields, including drug delivery. The interplay between substrate properties, surface interactions, enzyme reactions, and asymmetry opens new horizons for further exploration of chemotaxis in biochemical systems.   

Non-central forces in microgel colloidal crystals

Microgels are stimuli-sensitive colloids formed by cross-linked polymer networks and are a good model system for soft colloids. Poly(N-isopropylacrylamide) microgel copolymerized with acrylic acid (AAc) synthesized with APS or KPS initiators features a counterion cloud bound electrostatically to the charge groups at the particle surface. In concentrated suspensions, the percolation of the cloud causes a sharp increase of the osmotic pressure of the suspension that can overcome the bulk modulus of the microgels and, as a consequence, triggers their isotropic deswelling. It has been shown that for colloidal crystals of hard colloids, non-central forces dominate the interparticle interactions due to the presence of free counterions. Consequently, models that rely on the addition of pair-wise potentials, such as the DLVO model, can no longer sufficiently describe the system. Given the presence of free counterions in suspension, the microgel-microgel interaction is found to be non-central and analogous to the case of slightly charged hard colloids. In our experiment, at ζ~1.5 and ζ~1.3, the crystalline state of the pNIPAM-AAc microgels with a fluorescent core is studied at pH < 4 and pH = 7, corresponding to ACC being uncharged and charged, respectively. By measuring the normal modes using confocal microscopy with particle tracking, we extract the dynamic matrix of the crystal and the force constants of 1st neighbors and 2nd neighbors, as well as the elasticity constants of the crystal. The results turn out contradicting to the Cauchy relation, which indicates the non-central nature of the interaction force. In addition, we notice that F⁽²⁾_xx ~ -1/2 F⁽¹⁾_xx, indicating that the displacement of the 1st neighbor attracts the 2nd neighbor, contrary to the central force prediction. Our results show that the counterions of microgels with surface charge control both their swelling and their interaction at high concentrations. These findings must be considered for models of the swelling and phase behavior of soft colloids.   

Extinction and growth on an inhomogeneous Seascape

The effects of noise and nonuniformity on dynamics of populations are relevant and timely subjects of investigation. One form of variation is the time dependence of the reproduction rate (fitness), referred to as seascape noise; another is time-independent intrinsic dependencies of fitness on location (in the parlance of statistical physics, corresponding to annealed and quenched disorder, respectively). The former was studied recently and demonstrated to lead to novel universality classes for extinction and growth. To reduce the gap between this theoretical model and reality, we develop a new formalism for seascape noise where growth and migration parameters are inhomogeneous. In this formalism, we consider several subpopulation classes: each class consists of patches with similar properties, but patches for different classes are different. Employing a generalized mean-field approach, we self-consistently find distributions for numbers of each subpopulation in steady-state. Interestingly, we find that extinction is characterized by a critical exponent which depends on the characteristics of the subpopulation with the largest noise-to-migration ratio, regardless of the relative size of this subpopulation. Growth is now governed by a generalized Richards law, with an effective exponent varying with population size.   

Optimal transport and anomalous thermal relaxations

We study connections between optimal transport and anomalous thermal relaxations. A prime example of anomalous thermal relaxations is the Mpemba effect, which occurs when a hot system overtakes an identical warm system and cools down faster. Conversely, optimal transport is a resource-efficient way to transport the source distribution to a target distribution in a finite time. By resource-efficient, i.e., optimal transport, what is often meant is transport with the least amount of entropy production, and this is the definition we will use. Our paradigm for a continuum system is a particle diffusing on a potential landscape, while for a discrete system, we use a three-state Markov jump process. The Mpemba effect is generically associated with high entropy production in the continuous case. At large yet finite times, the system evolution toward the target is not optimal with respect to entropy production. However, in the discrete case, we show that for specific dynamics, the optimal transport and the strong variant of the Mpemba effect can occur for the same relaxation protocol.   

Optimizing data-driven learning of system reconstructions from incomplete low-dimensional observables

It is frequently of interest to reconstruct the state of a high-dimensional dynamical system that is implicitly observed through an incomplete set of low-dimensional variables. Under certain technical conditions, there exist theoretical guarantees on the existence of smooth and bijective maps between embeddings of the low-dimensional observables and the high-dimensional state of the system. This presents the opportunity to reconstruct high-dimensional system states from low-dimensional observations. In previous work, we combined Takens’ Delay Embedding Theorem with numerical universal function approximators to reconstruct the atomic coordinates of molecular systems from scalar time series observations of the molecular head-to-tail distance accessible to microscopy measurements. Takens’ Theorem is, however, silent on the optimal choice and processing of low-dimensional observables. This leaves open a key practical issue: How can observables be chosen and organized to maximally embed information available about system dynamics? In this work, we demonstrate how use of multiple observable streams, incorporation of prior knowledge of system structure, and use of multiple time delays can optimize reconstruction accuracy. In a chaotic pendulum model, we demonstrate how observable choice alters reconstruction quality and how multiplexing of observable streams can improve reconstruction fidelity. In molecular simulations of a C₂₄H₅₀ polymer chain, we show how incorporating prior knowledge alters reconstruction performance and we relate these choices to dynamical properties such Lyapunov exponents. In a Lotka–Volterra model of predator-prey dynamics, we show how dynamics evolving on multiple time scales are best reconstructed when multiplexing observable time delays on multiple time scales. We extract actionable guidelines from these studies that can be used to improve reconstruction and prediction quality for arbitrary dynamical systems in diverse fields including epidemiology, climatology, and econometrics.

    

Data-Driven Solution of the Inverse Problem of Classical Statistical Mechanics

Statistical mechanics aims to relate observable macroscopic properties of matter with its underlying microscopic structure via appropriate averaging. The central property which facilitates such averaging is the probability density ρ(r) of finding a particle near position-vector r. Finding ρ(r) can therefore be viewed as the direct problem of statistical mechanics. One way to find ρ(r) is by minimising the free energy the many-body system, expressed as a functional of ρ(r). Approaches based on this general idea are collectively known as Density Functional Theories (DFTs). In this talk we discuss DFTs of classical systems only (as opposed to quantum DFTs, which we do not consider here). 

Usually DFT approximations are interpretable and amenable to computation. This explains the popularity of DFTs across fundamental applications of statistical mechanics. However therein lies the principal challenge of using DFTs in engineering: one must know (or postulate) the density functional of the system under consideration. This is highly challenging in realistic scenarios encountered in, e.g., biology, nanotechnology and chemical engineering. Our talk aims to address this challenge. 

We pose the inverse problem of statistical mechanics: given particle data, characterise the free energy functional of the system. We then solve it using Bayesian reasoning. In the process, we develop a data-efficient learning algorithm that automates the construction of approximate free-energy functionals from small amounts of simulation data. On output the user gets a probability distribution over DFTs with full uncertainty quantification, allowing one to scale the description to system sizes far beyond the simulation capabilities. Our approach leverages modern computational capabilities in a physics-constrained framework to learn the free energy in a data-driven automated fashion. We validate our algorithm by consideing classical particle systems with excluded volume interactions. Such systems are ubiquitous in nature, while being highly challenging in terms of free energy modeling. We demonstrate that with appropriate particle data we can learn both the canonical and grand-canonical free-energy representations of such systems. Extensions to more complex and higher-dimensional systems are conceptually straightforward.   

Templated dynamical phases in biased ensembles of pulsating active matter

Using a recently introduced model of pulsating active matter we investigate its rare dynamics at high density conditions with respect to local and global order parameters in its size distribution. We find that for small systems these result in emergent dynamical phases that generate cycling, arrested or wave behavior as a function of box geometry and bias with respect to the order parameter. We ascribe cycling and arrested transitions to boundary effects imposed by the box geometry that alter its average packing properties in the steady state, and which remarkably `templates' the nature of the dynamical transition. Wave behavior, on the other hand, depends on an additional minimum box length constraint that builds on top of these packing conditions. We explain the emergence of arrested or cycling states from the perspective of a master curve between a global order parameter and average repulsion which hold generically for different box geometries, number of particles and densities.   

Channeling quantum criticality

We analyze the effect of decoherence, modelled by local quantum channels, on quantum critical states and we find universal properties of the resulting mixed state's entanglement, both between system and environment and within the system. Renyi entropies exhibit volume law scaling with a subleading constant governed by a "g-function" in conformal field theory (CFT), allowing us to define a notion of renormalization group (RG) flow (or "phase transitions") between quantum channels. We also find that the entropy of a subsystem in the decohered state has a subleading logarithmic scaling with subsystem size, and we relate it to correlation functions of boundary condition changing operators in the CFT. Finally, we find that the subsystem entanglement negativity, a measure of quantum correlations within mixed states, can exhibit log scaling or area law based on the RG flow. When the channel corresponds to a marginal perturbation, the coefficient of the log scaling can change continuously with decoherence strength. We illustrate all these possibilities for the critical ground state of the transverse-field Ising model, in which we identify four RG fixed points of dephasing channels and verify the RG flow numerically. Our results are relevant to quantum critical states realized on noisy quantum simulators, in which our predicted entanglement scaling can be probed via shadow tomography methods.   

Fast and precise promoter architectures in and out of equilibrium

Drosophila embryogenesis is controlled by a set of genes expressed sequentially at precise times and locations. To achieve this, the gene must make a fast and accurate readout of a given transcription factor concentration. We explore how different promoter architectures encode the speed and precision with which this readout takes place. Using the framework of sequential hypothesis testing applied to continuous-time Markov chains with concentration-dependent transition rates, we find the classes of circuits minimizing mean readout time, with and without an equilibrium constraint, and report on their properties.   

Investigation of Landauer's principle in a qubit erasure protocol at finite temperature.

More than sixty years ago, Landauer’s principle demonstrated a fundamental link between information theory and classical thermodynamics. The principle states that any irreversible informational process that occurs in a system is inevitably accompanied by the dissipation of heat into the environment. The principle was experimentally verified in the last decade with many different systems, including a trapped colloidal particle, a single atom, and a micromechanical oscillator. However, an absolute quantum version of the idea is still under debate and has attracted researchers from many different areas. Specifically, we have witnessed a renewed interest in this topic since the recent emergence of the area of quantum thermodynamics. In the present work, we propose a quantum information erasure protocol and analyze its implications in light of Landauer’s principle. The protocol relies on the use of the degeneracy in the degrees of freedom of a structured qubit-like reservoir to erase a given information stored in a qubit memory.  In this perspective, we can explicitly calculate

the change in the von Neumann entropy of the system, along with the heat dissipated to the degrees of freedom of the environment. Also, we provide an experimental testbed for the protocol based on an all-optical circuit with only linear optical devices. In this case, the polarization degree of freedom encodes the information to be erased in the memory, and the path degree of freedom plays the role of the reservoir. Likewise, a quantum circuit model is provided so that the steps of the erasure can be reproduced in other quantum platforms. As such, the model provides a simple and practical architecture for the realization of tests of the Landauer principle in a quantum-mechanical scenario.   

Anomalous Diffusion in Complex Environments: The Counterintuitive Influence of Obstacles in the Presence of Quenched Disorder

Anomalous diffusion where the mean squared displacement (MSD) does not grow linearly with time t and instead scales as t^α with 0<α<1 has been observed in living cells, glassy materials and many other systems. Counter-intuitively, we show that when the transport is anomalous, presence of obstacles can increase mobility. Simply said, the motion is faster when obstacles are present. We employ analytical and simulation tools to study this surprising effect. Our model consists of a random walker on top of a two dimensional lattice affected by traps or local areas of arrest that cause it to wait after each step. When the waiting times are taken from a heavy-tailed distribution this results in anomalous diffusion. We explore this scenario integrated with a quenched disorder of immobile obstacles. Two popular models of temporal disorder are considered: Continuous Time Random Walk, i.e. representative of annealed disorder, and Quenched Trap Model that represents the quenched disorder case. We show that while in the presence of annealed disorder of waiting times the response to external force decreases with the number of obstacles, it is the quenchness of the waiting times that gives rise to the observed exhibition of motion.   

Temperature replica exchange Gaussian accelerated molecular dynamics (T-Rex-GaMD): Improved enhanced sampling and energetic reweighting of biomolecules

Gaussian accelerated molecular dynamics (GaMD) provides enhanced sampling and energy reweighting of biomolecules. GaMD works through adding a harmonic boost potential, defined by a force constant and a threshold energy, to smooth the potential energy surface and accelerate sampling between the various states of a biomolecular system separated by large energy barriers. Previously, GaMD has been combined with replica exchange algorithms, resulting in force constant Rex-GaMD and threshold energy Rex-GaMD. Recently, this approach has been expanded to include parallel tempering, allowing exchanges at various temperatures. This new method, Temperature Rex-GaMD (T-Rex-GaMD), can exchange any combination of replicas defined over a range of different values of the force constant, energy threshold, and temperature. Our hope is that the use of high-temperature replicas, in addition to force constant and energy threshold replicas, will lead to accelerated sampling of interesting biomolecular systems with multiple different conformational states separated by large energy barriers, all while maintaining the accuracy of energetic reweighting to recover the true free energy profile. Successful T-Rex-GaMD simulations on test systems alanine dipeptide, chignolin, and HIV protease showcase its potential for advancing the study of biomolecular free energy profiles and providing profound insights into complex biological molecules.   

Revisiting the shepherding problem: An exercise in agent-based modelling

Revisiting the herd control problem, we propose a minimalistic protocol for controlling and guiding animal flocks towards a stationary target in a predator-prey framework. Our model considers stochastic activation of the herd (prey) from an initially static (grazing) state to motion on exposure to a predator. The information about the predator's presence propagates across the herd, leading to activation and, eventually, collective motion in a direction dictated by inter-prey alignment interactions and repulsive interaction with the predator. However, the herd moves intermittently, i.e., in a stop-and-go fashion often observed in livestock. The direction of movement of the herd is found to be highly correlated to the direction of the first activated agent relative to the predator. We determine the probability of a prey agent being activated first in the herd as a function of its position relative to the predator and surmise the predator's optimal path to drive the herd towards the target location. The analytical expression for the probability conforms to the results obtained from the agent-based simulations and can provide insights into a simple approach to herd control.   

Quantum coherence enhancement by the chirality-induced spin selectivity effect in the radical-pair mechanism

Avian magnetoreception is the ability of migratory birds to navigate using the Earth's magnetic field. The underlying biophysical mechanism relies on the spin dynamics of radical pairs (RP). The medium in which this reaction occurs is chiral that results in Chirality-Induced Spin Selectivity (CISS) effect affecting radical pair creation and recombination. In our work, we have studied the influence of CISS on the compass sensitivity and quantum coherence. We find that it enhances sensitivity and counters dipolar interaction's adverse effects. We observe significant compass sensitivity even under spin relaxation. Further, we studied CISS's impact on quantum coherence within the RP system. Interestingly, global quantum coherence correlates strongly with the compass signaling state yield. Our study considered simplified and comprehensive models, including up to eight nuclei. In the future, we plan to investigate the structural aspects of the medium during electron transfer, offering deeper insights into the overall radical pair mechanism. The study highlights the importance of harnessing the CISS effect in artificial quantum systems to achieve sustained quantum coherence.   

Physiological concentrations of calcium interact with alginate and extracellular DNA in the matrices of Pseudomonas aeruginosa biofilms to impede phagocytosis by neutrophils

Biofilms develop distinct mechanical characteristics that depend on their predominant matrix components. These matrix components may be produced by microbes themselves or incorporated from the host environment.  P. aeruginosa is a human pathogen that forms robust biofilms that extensively tolerate antibiotics and effectively evade clearance by the immune system.  Two of the important bacterial-produced polymers in the matrices of P. aeruginosa biofilms are alginate and extracellular DNA (eDNA), both of which are anionic and therefore have the potential to interact electrostatically with cations.  Many physiological sites of infection contain significant concentrations of the calcium ion (Ca²⁺). We investigated the structural and mechanical impacts of Ca²⁺ supplementation in alginate-dominated biofilms grown in vitro and evaluated the impact of targeted enzyme treatments on clearance by immune cells. We used multiple particle tracking microrheology to evaluate the changes in biofilm viscoelasticity caused by treatment with alginate lyase and/or DNAse I.   Our results suggest that the presence of Ca²⁺ drives the formation of structurally and compositionally discrete microdomains within the biofilm through electrostatic interactions with the anionic matrix components eDNA and alginate. Further, we observed that these structures serve a protective function as the dissolution of both components is required to render biofilm bacteria vulnerable to phagocytosis by neutrophils.   

Simulations validate a membrane-mechanical hypothesis for the action of an antiviral protein

IFITM3 is a protein with broad-spectrum anti-viral activity, the exact mechanism of which is not fully known. Experimental studies have pointed to the critical role of its amphipathic helix (AH) domain in inhibiting viral infection. Using molecular dynamics simulations of planar lipid bilayers, in conjunction with a novel theoretical framework for determining mechanical properties of molecular species in multi-component membranes, we determine the intrinsic curvature and thickness preference of IFITM3's AH. We then use continuum modeling to generate simulations of membranes in the hemifusion diaphragm geometry, substantiating the hypothesis that IFITM3 acts by inhibiting the fusion of the viral envelope and late endosomal membranes through stalling this intermediate step in the fusion pathway, driven by the mechanical characteristics of its AH domain.   

Electronic Interactions of DNA nucleobases with Single-Layer Ti3C2 MXene and Graphene: DFT and NEGF Studies

Recently, 2D MXenes have emerged as promising alternative materials for DNA nucleobase detection. A recent molecular dynamics simulation study using Ti₃C₂ MXene nanopores showed its potential for detecting nucleobases based on physical features such as ionic current and dwell time. However, molecular dynamics simulation can’t capture the electronic interactions between nucleobases and Ti₃C₂ which are very crucial for nucleobase detection. We investigated the electronic interaction of DNA nucleobases [adenine (A), guanine (G), thymine (T), and cytosine (C)] with single-layer Ti₃C₂ MXene using vdW-corrected DFT and NEGF methods. All calculations were benchmarked against graphene. We showed that depending on the initial vertical height of a nucleobase above the Ti₃C₂ surface, two interaction mechanisms are possible, namely: physisorption and chemisorption.  For graphene, DNA nucleobases always physisorped onto the graphene surface irrespective of the initial vertical height of nucleobase above graphene sheet. The PBE+vdW binding energies for graphene are high (0.55 – 0.74 eV) and follow the order G > A > T > C, with adsorption heights in the range 3.16 – 3.22 Å, indicating strong physisorption. For Ti₃C₂, the PBE+vdW binding energies are relatively weaker (0.16 – 0.20 eV) and follow the order A > G = T > C, with adsorption heights in the range 5.51 – 5.60 Å indicating weak physisorption. The binding energies for chemisorption follow the order G > A >T > C, which is the same order for physisorption. The binding energy values (5.3 – 7.5 eV) indicate very strong chemisorption (~40 times larger than the physisorption binding energies). Furthermore, our band structure and electronic transport analysis showed that for physisorption, there are neither significant variation in band structure nor modulations in the transmission function and device density of states (DDOS). The relatively weak physisorption and strong chemisorption shows that Ti₃C₂ might not be capable of identifying DNA nucleobases using the physisorption method.   

Simulation of X-ray photoelectron spectroscopy in atoms, molecules, and clusters: Core-electron excitation from ab initio many-body approach

X-ray photoelectron spectroscopy (XPS) technique, measuring directly core-electrons binding energies (BEs), provides information about electronic structure, chemical bonding, and stoichiometry for molecules/solids. This work presents the benchmark study of core electrons BEs in noble gas atoms between theories, including density functional theory (DFT), Hartree-Fock (HF) and many-body theory perturbation theory (GW approach) against experiments first, pointing out significant improvement of computed BEs from HF/DFT to GW. Furthermore, XPS of noble gas clusters with 3000 atoms were studied with embedded many-body theory to estimate the environmental polarization effect on relative BEs (chemical shifts). While the polarization energy remains consistent for different core orbitals within a given atom, it varies with the ionized atom's position in the cluster. An analytical formula derived from classical electrostatics accurately describes these polarization effects, aligning well with experimental XPS for noble gas clusters. Finally, by investigating the core-electron excitation in carbon 1s among various molecules, we found that the main contribution to chemical shift comes from classical electrostatic interaction and is one order of magnitude larger than the correlation effects.   

Applying the Potential Energy Landscape Formalism to a Flexible Water Model

The potential energy landscape (PEL) formalism is a valuable theoretical approach within statistical mechanics to describe supercooled liquids and glasses. Here, we use the PEL formalism and computer simulations to study the statistical properties of the PEL of a flexible water model. We perform extensive molecular dynamics (MD) simulations of H₂O using the flexible q-TIP4P/F water model over a wide range of volumes and temperatures. We find that the PEL of the q-TIP4P/F water model can be accurately described by a Gaussian landscape. Within the Gaussian landscape, we calculate an equation-of-state (EOS) and the find that EOS predicted from the PEL is in excellent agreement with the density (ρ(T)) and pressure (p(V)) obtained from the MD simulations. Using the EOS obtained from the PEL, we estimate the liquid-liquid critical point (LLCP) for q-TIP4P/F H₂O, is located at P_c≈140 MPa, T_c ≈ 195 K, and ρ_c ≈ 1.02 g/cm³, consistent with the data from the MD simulations. Finally, we compare the PEL properties of the flexible q-TIP4P/F water model with those reported previously for rigid water models (SPC/E and TIP4P/2005). Interestingly, we find that the PEL properties for the flexible and rigid water models are very similar.   

Spectral correlations of photon-pairs: a comparison of SPDC-based sources

The generation of spectrally uncorrelated single photons and spectrally correlated photon pairs is crucial for catering to various applications in quantum technology. While spectrally correlated biphotons improve versatility, measurement accuracy and security in quantum communication and networking systems, spectrally pure photons can be used for quantum key distribution systems and linear optical quantum computational networks. The joint spectral intensity (JSI) measurements of spontaneous parametric downconversion (SPDC) based photon sources give a holistic view of correlations present in the generated photon pairs. In this work, we compare JSI measurements of three sources which include type-0 fiber coupled Zn indiffused MgO:ppLN ridge waveguide (4 cm), type II MgO:ppLN ridge waveguide (1.69 cm) and free-space ppKTP crystal (3 cm). We observed the JSI plots of two sources, broadband (~46 nm) in type-0 interaction and narrowband (~2.4 nm) in type II interaction, are negatively inclined revealing strong spectral correlations. The JSI plot of type II SPDC based ppKTP crystal also depicts spectral correlations in an even narrower band (~0.62 nm) owing to longer length of the crystal. However, careful selection of aperture size placed after the crystal can help in the generation of spectrally pure single photons as well. The results show that the width of observed JSI depends on the inherent emission bandwidth of photon sources which further depends on the dispersion properties of the material, length of the crystal/waveguide, and filter bandwidths used in the experiment.   

High fidelity distribution of entanglement on metropolitan fibres using warm atomic vapors

The efficient distribution of entanglement over quantum channels is necessary for long distance quantum networks. Here, we present a high spectral brightness source of bichromatic, polarization entangled photons, based on spontaneous four-wave-mixing in a warm rubidium vapor. The source produces pairs with one of the photons in the telecom O-band and the other at 795-nm. Furthermore, we report on experiments performed with the source using dark fibers in New York City, demonstrating its compatibility with existing telecom infrastructure.   

Driven-Dissipative Bose-Einstein Condensation and the Upper Critical Dimension

Driving and dissipation can stabilize Bose-Einstein condensates. Using Keldysh field theory, we analyze this phenomenon for Markovian systems that can comprise on-site two-particle driving, on-site single-particle and two-particle loss, as well as edge-correlated pumping.

Above the upper critical dimension, mean-field theory shows that pumping and two-particle driving induce condensation right at the boundary between the stable and unstable regions of the non-interacting theory. With nonzero two-particle driving, the condensate is gapped. This picture is consistent with the recent observation that, without symmetry constraints beyond invariance under single-particle basis transformations, all gapped quadratic bosonic Liouvillians belong to the same phase.

For systems below the upper critical dimension, the edge-correlated pumping penalizes high-momentum fluctuations, rendering the theory renormalizable. We perform the one-loop renormalization group analysis, finding a condensation transition inside the unstable region of the non-interacting theory. Interestingly, its critical behavior is determined by a Wilson-Fisher-like fixed point.   

Unidirectional flow of flat-top solitons

We numerically demonstrate the unidirectional flow of flat-top solitons when interacting with two reflectionless potential wells with slightly different depths. The system is described by a nonlinear Schr"{o}dinger equation with dual nonlinearity. The results show that for shallow potential wells, the velocity window for unidirectional flow is larger than for deeper potential wells. A wider flat-top solitons also have a narrow velocity window for unidirectional flow than those for thinner flat-top solitons.   

Non-linear transport in polariton graphene

Due to its unique electronic and optical properties, graphene is widely used in state-of-the-art technology as well as fundamental material science.  Moreover, there are extensive studies of the topological and transport characteristics of artificially created honeycomb lattices. In the present work we consider the exciton-polariton analog of a single-layer graphene, where the spatial inversion symmetry is broken due to embedding of triangle-shaped defects. It leads to asymmetric scattering.  Our numerical simulations using the corresponding Gross-Pitaevskii equation show presence of the ratchet motion as a consequence of the scattering on defects. We study various realizations of defect profiles which can be realized experimentally.  In addition, results of our numerical calculations are in agreement with analytical predictions.    

Inverse-designed nanophotonic resonators for cavity QED in diamond

Nanophotonic platforms are key ingredients for scalable solid-state quantum information processing. In particular, diamond nanophotonic resonators are central instruments for the realization of quantum memories. Over the past decade, inverse design has emerged as a promising method for various photonics applications, yielding non-intuitive device geometries that exhibit a comparable if not superior performance relative to their conventional counterparts. However, as a material, diamond has remained elusive for inverse design methodologies due to limitations in processing and fabrication. The recent advent of thin-film diamond on insulator material now allows for the full potential of inverse design. In this study, we employ photonic inverse design methodologies to explore novel resonator geometries intended for thin-film diamond platforms.

(*)The authors wish to thank the SPINS Photonics team, whose help has been vital for this research. All devices in this publication were designed with fdtd-z, SPINS Photonics' open-source, GPU-accelerated FDTD simulation engine.   

Kondo effect in Twisted Bilayer Graphene

Graphene has a lot of interesting properties. However, it's low density of states near the charge neutrality point means that it can not show a Kondo effect under generic circumstances. Depending on the hybridization, magnetic impurities embedded on its surface remain unscreened even at the lowest temperatures. We show that this is no longer the case in magic angle twisted bilayer graphene. By studying the effective Bistritzer-MacDonald model, we capture features of the density of states at low energies as a Dirac cone flanked by van Hove singularities. As the two layers of graphene are twisted toward the magic angle, these logarithmic van Hove singularities approach each other in energy, and pinch off the pseudogap associated with the cone, forming a higher order order power law singularity at the magic angle. The enhanced density of states at the Fermi energy plays a critical role in the re-entrance of strongly correlated phenomena like the Kondo effect due to a magnetic impurity. By studying the resulting quantum impurity physics using perturbative and numerical renormalization group methods, we find that at zero temperature the impurity is only Kondo screened precisely at the magic angle. We also offer predictions of highly nontrivial behavior at finite temperatures relevant to experiments, due to the complex interplay between Dirac, van Hove, and Kondo physics.   

Topological and correlated states in spin-orbit coupled graphene-insulator heterostructures

In this work, we theoretically study the topological and correlated states in band-aligned graphene-insulator heterostructures systems including spin-orbit coupling (SOC) effects. Typical examples include graphene-transition metal dichalcogenide (TMD) heterostructures. On the one hand, the band edges are energetically close to the Dirac point in graphene, which may induce charge transfer between the two layers. The transferred charges in the TMD layer may form long-wavelength electronic crystal at sufficiently low carrier densities, which exerts a superlattice potential to the graphene layer through interlayer Coulomb coupling. As a result, the Fermi velocity of Dirac fermions in graphene would be reduced due to scatterings by the superlattice Coulomb potential, which thus boosts electron-electron interaction effects in graphene layer. On the other hand, TMD can induce proximity SOC in graphene, which would generate topological subbands with nonzero valley Chern numbers if considering effects of both SOC and long-wavelength Coulomb potential. We further study the interacting ground states based on unrestricted Hartree-Fock calculations including the Coulomb screening effects from the remote bands, and find a variety of symmetry-breaking ground states. Within the same theoretical framework, we also investigate the electronic structures, topological properties, and interaction effects in coupled bilayer graphene-TMD heterostructures.   

Probing Interface Separation for Fermi-Level Depinning in 2D Semiconductor-Metal Contacts

In recent studies of metal-semiconductor (2D) contacts, parasitic contact resistance remains problematic [1]. Fermi-level pinning is a major contributor to this issue. Various strategies have been explored to mitigate this problem, such as incorporating dielectric or semi-metallic layers at the interface to isolate the semiconductor layer and prevent the penetration of metal-induced gap states (MIGS) into the bandgap. However, the characterization of these methods in terms of interface distance is yet to be explored. Our study investigates the impact of interface separation on contact properties, including Schottky barrier heights (SBH), tunnel barrier heights (TBH), and MIGS. We aim to find the optimal separation to eliminate the MIGS effect and de-pin the Fermi level. We assess various metals (Ag, Au, Pt) when interfaced with the 2D semiconductor WS₂, exploring the potential of inducing SBH polarity based on work functions. Additionally, we present a simple first-principles approach for calculating band bending in semiconductor-metal contacts. Our study underscores the crucial role of interface separation in achieving Fermi-level depinning, which is essential for enhancing the performance of metal-semiconductor interfaces.

References:

[1]. Ghaffar, A., Ganeriwala, M. D., Hongo, K., Maezono, R., & Mohapatra, N. R. (2021). Insights into the Mechanical and Electrical Properties of a Metal–Phosphorene Interface: An Ab Initio Study with a Wide Range of Metals. ACS omega, 6(11), 7795-7803.   

Hopf Semimetals: Protected Gapless Phases from Unstable Homotopies

We construct and explore two-band topological semi-metals in different spatial dimensions that are protected by unstable homotopies. These semimetals that generically host nodal lines are dubbed "Hopf semimetals'', inspired by the example of such phases realized in four dimensions arising from maps from the three-torus T³ (Brillouin zone of a 3D crystal) to the two-sphere S² . In the four-dimensional example, a surface enclosing such a nodal line in Brillouin zone carries a Hopf flux. These 4D semimetals show a unique class of surface states: while some three-dimensional surfaces host gapless Fermi-arc states and drumhead states, other surfaces have gapless  Fermi surfaces. Gapless two-dimensional corner states are also present at the intersection of three-dimensional surfaces. We also demonstrate such semimetals realized in three dimensions in chiral class AIII, that arise from the unstable homotopies of maps from T² (Brillioun zone of a 2D crystal) to S¹. These 3D semimtals also hosts nodal lines, accompanied by a rich collection of surface states including drumhead type. This work provides a new framework to realize protected nodal line semimetals particularly in synthetic quantum systems such as cold atoms and photonic systems.   

Cellular Dynamical Mean Field Study of Intra-Unit-Cell Charged Nematicity in Cuprates

A recent experiment on hole-doped Bi₂Sr₂CaCu₂O₈, one of the materials of the cuprate family, finds a splitting in the energy levels of the oxygen orbitals of the CuO₂ unit cell. This observation has been predicted to occur due to spontaneous intra-unit-cell orbital ordering, caused by the Coulomb interaction (denoted by Vpp) between electrons of 2p⁶ orbitals of neighboring planar oxygen atom. In our work, we verify this prediction by analyzing the three-band Hubbard (or Emery) model using cluster dynamical mean field theory (CDMFT). We find that indeed the system develops intra-unit-cell charged-nematic order (the name by which this orbital ordering is known) when the Vpp interaction is finite. However, the crucial interplay of this interaction with other ones in the unit cell, such as the oxygen on-site interaction (Up) and the Coulomb interaction between electrons of Copper and oxygen orbitals (Vpd), results in robust nematicity only at large hole doping. We also compare our findings with those from Hartree-Fock mean field theory (MFT). The predicted nematicity is also much larger than the observed signature, casting doubts on the actual cause of the splitting.   

Understanding the Dynamic Response of Nanoelectromechanical Sensors to Ultra-High Purity Gases: Implications for Gas Sensing and Surface Chemistry Study

Nanoelectromechanical sensors are exceptionally sensitive for gas detection and mass measurement. To detect gases effectively, it's crucial to comprehend the interaction between analytes and sensors, even in non-specific adsorption cases. This study investigates the impact of pulses of ultra-high purity (UHP) gases on nanoelectromechanical resonators. The study examines frequency shifts in uncoated microcantilevers, revealing three distinct temperature regimes. Above 200K, positive frequency shifts occur, while between 200K and 100K, temporary frequency dips are observed. Below 100K, a continuous negative frequency drift appears, along with negative frequency shifts upon gas introduction. These shifts result from the interplay of squeeze film damping and moisture adsorption on the cantilever, with the gases themselves (argon or helium) having a minimal role in the negative frequency shifts. The study elucidates these findings using analytical and finite element method (FEM) simulations, capturing the kinetics of adsorption-desorption processes under varying partial pressures and temperatures. This research provides a comprehensive understanding of how mechanical resonators respond to UHP gases and their impurities, with broad implications for gas sensing using mechanical resonators.   

Density Wave order and Topological superconductivity in Pd2ZrGa

Pd-based Heusler alloys demonstrate intriguing transport properties, including superconductivity and density wave order, due to the presence of a van Hove Singularity at the L point. Through first-principle calculations, we investigate the properties of Pd₂ZrGa, exploring both its primitive L2₁ (FCC) and conventional cubic Heusler structures. The divergent behavior seen in the Pd-d and Zr-d states at the Fermi level, coupled with the notable enhancement in the Ga p states, suggests that this occurrence is a consequence of lattice instability rather than magnetic instability. The phonon dispersion relation for Pd₂ZrGa exhibited an imaginary frequency for both the primitive and conventional cells, indicating lattice instability. We estimated the Fermi surface nesting function using a double delta integration over the full Brillouin zone, revealing the presence of small parallel surfaces at the M and R points, suggesting a weak nesting behaviour. The frequency dependence of the mode-resolved and total α²F(ω) indicates that low-energy phonon is involved in electron scattering, resulting in an estimated λ value of 1.37 and T_c value of 4.3 K and log(ω_D) = 62 signifies strong coupling and phonon-mediated superconductivity. Further, Dirac point at the Fermi level and four additional points within ±4 meV points toward exotic electronic properties in Pd₂ZrGa.   

Dynamically confined superwires as stable electron guides in the presence of lattice defects

Guiding electrons along a 2D material, in a similar fashion to optical waveguides, can be one way to control electron transmission and improve electronic and even quantum communication. It was recently found that electrons propagated in a 2D periodic superlattice potential, more specifically in a linear channel, stay in the channel for a long time. In these channels, namely "superwires", electrons are not mechanically confined with a high barrier of potential, and this 1D localization occurs dynamically. We demonstrate that by altering parameters such as lattice constant and shapes and heights of the potential, we can modulate allowed electron wavelengths in superwires. Furthermore, dynamic superwires can remain robust against vacancies, impurities, and disorder emerging from finite-temperature lattice vibrations. The stability analysis of dynamical channels in a realizable parameter regime is discussed. The suppressed electron-phonon interactions in the flat bands of high Brillouin zones imply reduced backscattering and may give rise to zero-resistivity transport. This stability of the dynamical localization of electrons against perturbations in a non-integrable system promises a novel way to control electron transport in nanoscale devices.   

Integration of perovskite based transparent conducting oxides on industrial substrates: the key role of the glass substrate properties and binary oxides seed layers

The discovery of conductivity and transparency in strongly correlated metals of the ABO₃ type has paved the way for the development of a new generation of indium-free transparent conducting oxides (TCOs). SrVO₃ (SVO), CaVO₃, SrNbO₃ and SrMoO₃ perovskite oxides have demonstrated comparable electrical and optical properties compared to Indium-Tin-Oxide (ITO), the standard material for TCO devices. Integrating these new TCOs materials on low-cost substrates for large-scale production remains a technological challenge because of the necessity of a compatible growth template for the crystalline growth of this new generation of TCOs, in contrast to ITO, which can be grown easily on industrial substrates and yet has good properties while being amorphous. 

In this presentation, we introduce an innovative approach to the integration of SVO thin films onto industrial substrates, such as glass or silicon. Our method uses a TiO₂ seed layer to facilitate growth and enhance crystalline quality of SVO films. The resulting thin films are polycrystalline in nature, exhibiting remarkably low electrical resistivity of about 400 µΩ.cm and good optical transparency exceeding 70%. These characteristics underline the immense potential of these films for a wide range of electronic applications.

Furthermore, our study also focuses on the intricate interplay between the chemical and thermal properties of the substrate, specifically glass, and their influence on the properties of SVO thin films. A deeper understanding of these substrate properties not only contributes to optimizing the growth process but also highlights the critical role of the substrate in achieving the desired properties of these new TCOs materials.   

Two strange metal regions in the doped triangular lattice Hubbard model

In recent years, the T-linear resistivity found at low temperatures, defining the strange metal phase of cuprates, has been a subject of interest. Since a wide range of materials have a scattering rate that obeys the equation ħ/τ = k_BT, the idea of a universal Planckian limit on the scattering rate has been proposed ¹. However, there is no consensus on proposed theories yet. In this work, we present our results for the strange metal phase in the triangular lattice Hubbard model obtained using the dynamical cluster approximation. We find two regions with T-linear scattering rate in the T-p phase diagram: one emerges from the pseudogap to correlated Fermi liquid phase transition, whereas the other is solely caused by large interaction strength.

 

[1] Bruin, J. A. N., H. Sakai, R. S. Perry, et A. P. Mackenzie. « Similarity of Scattering Rates in Metals Showing T-Linear Resistivity ». Science 339, no 6121 (15 février 2013): 804‑7. https://doi.org/10.1126/science.1227612.

    

Navigating the high-Tc Pairing Landscapes of Nickelates Superconductivity: an Experimentalist’s Perspective

While Ni¹⁺ of d⁹ electronic structure was introduced as a promising high-T_c superconductor that could resemble the Cu²⁺ state in cuprates, both theoretical calculations and experimental evidence suggest that d⁹ nickelate superconductivity is not the same as cuprates’ after all [1-4], and high-temperature superconductivity remains elusive in the d⁹ nickelate. Through the observation of dimensionality control [4] with the choice of dopants and rare-earth elements, we postulate a much wider pairing landscape in the d⁹ nickelate superconductivity which can be accessible by, for example, synthesizing a more two-dimensional infinite-layer nickelate – the Ba-doped LaNiO₂ which should make a closer resemblance to the single-band Hubbard model. In this presentation, we will present our ongoing experimental effort in synthesizing ambient-pressure high-T_c d⁹ nickelate superconductor, as well as our recent advancement in the growth of high-quality samples with a large residual-resistivity-ratio RRR up to ~ 13, and low residual resistivities at the order of few ~ 10 µΩ.cm.

 

[1] K. W. Lee, W. E. Pickett, Phys. Rev. B - Condens. Matter Mater. Phys. 70 (2004) 1-7

[2] L. E. Chow et. al., arXiv:2201.10038

[3] L. E. Chow et. al., arXiv:2204.12606

[4] L. E. Chow et. al., arXiv:2301.07606   

A First-principles study of Defects in Garnet Type Solid State Electrolyte Li7La3Zr2O12

The Garnet-type solid electrolyte Li₇La₃Zr₂O₁₂ (LLZO) is one of the most promising candidates for solid-state batteries. This study focuses on the cubic-LLZO due to its high ionic

conductivity. The primary goal here is to optimize the ionic conductivity in cubic-LLZO, and one strategy to attain this goal is by performing the native defect analysis of the cubic-LLZO. Density functional theory was employed to calculate defect formation energies for a range of point defects. We use a thermodynamically stable region of cubic-LLZO which can be broadly classified into two categories: O-rich/metal-poor and O-poor/metal-rich conditions. In O-poor conditions, defects such as O vacancies, La and Zr substitutions, and Li interstitials exhibit higher concentrations. Conversely, in O-rich conditions, Li, La, and Zr vacancies have higher concentrations. This work provides a fundamental understanding of the defects, facilitating further investigations into the conduction mechanism of cubic-LLZO under extreme conditions.   

Non-conservation of the valley density and its implications for the observation of the valley Hall effect

We show that the conservation of the valley density in multi-valley and time-reversal-invariant insulators is broken in an unexpected way by the electric field that drives the valley Hall effect. This implies that fully-gapped insulators can support a valley Hall current in the bulk and yet show no valley density accumulation on the edges. Thus, the valley Hall effect cannot be observed in such systems. If the system is not fully gapped then valley density accumulation at the edges is possible and can result in a net generation of valley density. The accumulation has no contribution from undergap states and can be expressed as a Fermi surface average, for which we derive an explicit formula. We demonstrate the theory by calculating the valley density accumulations in an archetypical valley-Hall insulator: a gapped graphene nanoribbon. Surprisingly, we discover that a net valley density polarization is dynamically generated for some types of edge terminations.   

Modeling Current Flow Through Copper with Surface Substituted Intermetallic Barrier Layers

With the continuous shrinking of integrated circuits comes the likewise size reduction of their interconnects. This comes at the cost of increased resistivity of the interconnects, being exacerbated by surface roughness which is required for adhesion to circuit components. The size of these interconnects, being on the order of a few nanometers, warrants consideration of electronic scattering effects to determine their transport behavior. We employ a first-principles approach using density functional theory (DFT) in conjunction with the Keldysh non-equilibrium Green's functions (NEGF) formalism to determine the electronic structure of nanoscale Cu interconnects with roughened intermetallic barriers under a bias voltage. Tangible properties such as conductance are recovered with NEGF-DFT. We hypothesize that introducing a barrier layer may mitigate electronic scattering at the roughened surface and recover some of the conductance that is lost when said roughness is introduced in a pure Cu interconnect. In particular, we probe barrier layers composed of Mn, Ni, Zn, Ag, Sn, and Cu3Sn with five distinct roughness configurations, where half of the top barrier layer atoms have been randomly removed. We also investigate the effects of a non-ideal interface by intermixing barrier layer atoms below the roughened surface, creating an "interphase" between the barrier and the Cu film.   

Spin orbit torque driven strong spin to charge conversion in all oxide interface

The behavior of spin orbit coupling (SOC) including special phenomenon (such as those showing the Rashba-Edelstein effect, Spin Galvanic effect, and others), which can only appear in low-dimensional materials and low-dimensional systems, interplays crucial role in between spin and charge currents. Also, effective spin orbit torque (SOT) can modulate magnetization in various ferromagnetic/non-magnetic bilayers. In this study, we studied spin orbit torque strength by measuring ferromagnetic resonance (FMR), and studied field induced magnetization switching as well. at all oxide structure of La_1-xCa_xMnO₃ (LCMO)/SrTiO₃ (STO) bilayer. Quasi 2-dimensional conducting surface of STO was prepared by Ar plasma treatment. After then, the magnetic oxide LCMO thin film was deposited by using Pulsed Laser Deposition (PLD) method. Further, possible origins of the spin orbit torque generated at the conducting interface between LCMO and STO will be discussed later. Such study on low-dimensional materials with comparable energy scales among kinetic energy, spin-orbit interaction, and magnetic field, which opens a new route to enhance nonreciprocal response and its functionalities in the emerging spin-orbitronics.   

Nernst effect in strange metals

The strange-metal state is a crucial problem in condensed matter physics highlighted by its ubiquity in many correlated systems [1, 2]. Its understanding could provide important insight into high-T_c superconductivity and quantum criticality. However, with the Fermi liquid theory failing in strange metals, understanding the highly unconventional behaviors has been a long-standing challenge. Fundamental aspects of strange metals remain elusive, including the nature of their charge carriers. In this talk, I will report our recent effort in this direction using the Nernst effect as a sensitive probe to the entropy of charge carriers [3]. Specifically, we identified a strongly enhanced Nernst response in the strange-metal state in a two-dimensional superconductor 2M-WS₂ [4]. A large Nernst coefficient comparable to the vortex Nernst signal in superconducting cuprates, and its high sensitivity to carrier mobility, are found when the system enters the strange-metal state from the Fermi liquid state. The temperature and magnetic field dependence of the giant Nernst peak rule out the relevance of both Landau quasiparticles and superconductivity. Instead, the giant Nernst peak at the crossover indicates a dramatic change in carrier entropy when entering the strange-metal state. The presence of such an anomalous Nernst response is further confirmed in other iconic strange metals, suggesting its universality and places stringent experimental constraints on the mechanism of strange metals. 

 

[1] P. W. Phillips, N. Hussey, and P. Abbamonte, Stranger than metals, Science 377, 169 (2022).

[2] S. A. Hartnoll and A. P. Mackenzie, Colloquium: Planckian dissipation in metals. Rev. Mod. Phys. 94, 041002 (2022).

[3] K. Behnia and H. Aubin, Nernst effect in metals and superconductors: a review of concepts and experiments. Rep. Prog. Phys. 79, 046502 (2016)

[4] Y. Yang, et al., Anomalous enhancement of the Nernst effect at the crossover between a Fermi liquid and a strange metal, Nature Physics 19, 379 (2023).   

Revealing the Charge Density Wave Proximity Effect in Graphene on 1T-TaS2

The proximity-effect, a phenomenon whereby materials in close contact appropriate each other’s electronic-properties, is widely used in nano-scale devices to induce electron-correlations at heterostructure interfaces. Commonly observed proximity-induced correlation-effects include superconductivity, magnetism, and spin-orbit interactions. Thus far, however, proximity induced charge density waves (CDW) have not been rigorously explored, primarily because of screening in 3D metals and defect scattering at interfaces. Here, we report the observation of a CDW proximity effect between graphene and the commensurate CDW in 1T-TaS₂ (henceforth called TaS₂ for brevity). Using scanning tunneling microscopy (STM) and spectroscopy (STS) together with theoretical modeling to probe the interface between graphene and a TaS₂ crystal, we demonstrate the existence of a proximity induced CDW within graphene. Furthermore, we observe that graphene modifies the band structure at the surface of TaS₂, by providing mid-gap carriers and reducing the strength of electron correlations there. We show that the mechanism underlying the proximity induced CDW is well-described by short-range exchange interactions that are distinctly different from previously observed proximity effects.   

Transport in periodically and aperiodically driven quantum systems

Recent research on periodically and aperiodically driven systems has revealed several novel, interesting non-equilibrium phenomena. Electric field-driven systems are a particularly important subclass from the perspective of Floquet engineering [1]. We study the non-trivial dynamics of the delocalized and localized phases [2] of a family of time-periodic and aperiodic electric-field-driven quantum systems in the non-interacting and interacting limits. In the presence of disorder, low-frequency periodic driving leads to subdiffusive transport in both the non-interacting and interacting limits. Next, the study of aperiodically (Fibonacci and Thue-Morse) driven systems shows anomalous transport in both the non-interacting and interacting limits.

We also study periodically and quasi-periodically driven long-range interacting systems. The interplay of periodic driving and long-range interaction results in a suppression of heating for a longer time if we tune the driving at a high frequency due to the frozen dynamics of the system. Interestingly, in the presence of discrete quasiperiodic driving (Fibonacci and Thue-Morse), the dynamics get slower with an increase in the range of interaction [3].

(1). Tiwari et al., arxiv: 2302.12271.

(2). Tiwari et al., Transport in periodically and aperiodically driven quantum systems, (manuscript under preparation).

(3). Tiwari et al., Dynamics of periodically and aperiodically driven long-range quantum systems, (manuscript under preparation).   

Phase Stability and Crystal Orientation of Low-Dimensional Hybrid Halide Perovskite Films grown by Chemical Vapor Deposition

Low-pressure chemical vapor deposition offers a scalable and low-cost route for the conformal deposition of perovskite materials. Using a multi-step CVD process, we demonstrate the versatility of the technique in the growth of phase stable 3D and 2D lead-halide perovskite films. An air-processed methylammonium lead iodide (MAPbI₃) planar single-junction solar cell was realized and maintains 85% of its performance up to 2 weeks in the open air. Using large organic cations, such as butylammonium (BA) and phenylethylammonium (PEA), CVD was used to grow BA₂PbI₄ and PEA₂PbI₄ and its mixed halides with chlorine. X-ray diffraction (XRD) and a non-integer dimensionality model of the absorption spectrum provide insights into the orientation of the crystalline planes of PEA₂PbI₄. Furthermore, detailed XRD analysis of the PEA₂PbI₄ film as a function of film thickness show the appearance of 2D powder diffraction spots, highlighting the different orientational properties of the perovskite planes. Contrary to BA₂PbI₄, temperature-dependent photoluminescence of PEA₂PbI₄ show a single excitonic peak throughout the temperature range from 20 – 350 K, highlighting its phase stability and lack of defect states. These results are further corroborated by temperature-dependent XRD analysis.   

Quasi two dimensional antiferromagnetism in square planar iridate Cs2Na2IrO4

In recent times, the study of iridates has gained major attention and the driving force has been the delicate interplay amongst various energy scales. The local environment of the Ir atom is one of the crucial factors in dictating the properties of such systems and the constant focus has been on Ir in the octahedral or tetrahedral environment. In this work we report the rare occurrence of square planar iridate Cs₂Na₂IrO_4. The structure consists of isolated IrO₄ planes, orthogonally oriented in consecutive layers. Our work involves the  detailed study of electronic and magnetic properties of Cs₂Na₂IrO₄, from first principles calculations. Microscopic magnetic exchange interactions and Wannier function analysis reveals the quasi two dimensional canted AFM ground state, despite the absence of long range structural connectivity which originates because of very weak spin-phonon coupling. The phonon modes analysis further sheds light on the origin of orthogonally placed IrO₄ moieties. Since Ir belongs to the 5d series, it is known to be substantially impacted by SOC. However due to the half-filled situation, the orbital magnetic moment is quenched in this case. Nevertheless we still obtain a large magneto-crystalline anisotropy which could be explained from the second-order perturbation theory. As a guiding tool to experimentalists, we also report preliminary work on muon active sites which is crucial in obtaining the magnetic structure of the system for future studies.    

Hidden magnetism uncovered in a charge ordered bilayer kagome material ScV6Sn6

Charge ordered kagome lattices have been demonstrated to be intriguing platforms for studying the intertwining of topology, correlation, superconductivity and magnetism [1-4]. The recently discovered charge ordered kagome material ScV₆Sn₆ [5] does not feature a magnetic groundstate or excitations, thus it is often regarded as a conventional paramagnet. In this talk, I will present the results of muon-spin rotation and magnetotransport experiments, uncovering an unexpected hidden magnetism of the charge order [6]. We observe a striking enhancement of the internal field width sensed by the muon ensemble, which takes place within the charge ordered state. More remarkably, the muon spin relaxation rate below the charge ordering temperature is substantially enhanced by applying an external magnetic field. Taken together with the hidden magnetism found in AV₃Sb₅ (A = K, Rb, Cs) [1-3] and FeGe [4] kagome systems, our results suggest ubiqitous time-reversal symmetry-breaking in charge ordered kagome lattices. This is substantiated by our very recent discovery of above room-temperature charge order and its magnetic response in a prototypical kagome superconductor LaRu₃Si₂ [7].

[1] B. Ortiz, et al. Phys. Rev. Lett. 125, 247002 (2020).

[2] C. Mielke III, D. Das, et al. and Z. Guguchia. Nature 602, 245-250 (2022).

[3] Z. Guguchia et al. Nature Communications 14, 153 (2023).

[4] J.-X. Yin et al. Phys. Rev. Lett. 129, 166401 (2022). 

[5] H.W.S. Arachchige, et al. Phys. Rev. Lett. 129, 216402 (2022).

[6] Z. Guguchia et al. arXiv:2304.06436v1 (2023). 

[7] I. Plokhikh, et al. and Z. Guguchia. arXiv:2309.09255 (2023).

Spin reorientation of the van der Waals magnet CrCl3 induced by high-pressure: MuSR and neutron diffraction study.

Researches on two-dimensional (2D) materials have attracted tremendous attention both from fundamental and applied sciences, accelerated by the discovery of graphene. Among a large number of 2D materials, chromium trihalides CrX₃ (X = Cl, Br, I) van der Waals (vdW) magnets have also raised a large interest due to the existence of many magnetic subtleties that cannot be simply explained by their magnetic and/or structural transitions.

Numerous studies were performed on CrI₃, but only a few have been reported so far on its analogue CrCl₃. The 2D vdW CrCl₃ compound is stabilized under a rhombohedral symmetry, consisting of 2D Cr layers arranged in a honeycomb web fashion and surrounded by octahedrally coordinated Cl, with weak vdW inter-layer coupling. This makes CrCl₃ an ideal system to study under external stimuli such as pressure or magnetic field, where new intriguing states of matter can be unveiled. Expectantly, studies of CrCl₃ under high pressure and room temperature have been reported. [1] However, its spin dynamics at low-temperature and high-pressure regimes remain unexplored. Motivated by the variability of the spin degree of freedom and spin dynamics under such conditions, we have performed muon spin rotation (mSR) and neutron powder diffraction (NPD) on ambient and hydrostatically pressured CrCl₃ up to 23 kbar down to 2 K. [2,3]

In this study, by incorporating the two techniques, we resolved a suppression of the magnetic ground state and a stronger relaxation rate by MuSR. Within the magnetically ordered states, a spin reorientation was also observed by NPD at high pressure. This further extends the investigation into the interplay between the inter-, and intra-layer coupling in the vdW magnet family. A linear extrapolation points toward the suppression of magnetism at about p_c= 30 kbar indicating the possible existence of a critical point at p_c. [3]

[1] Ahmad, A., et al. Nanoscale 12.45 (2020): 22935-22944.

[2] Forslund, O., et al. arXiv:2111.06246 (2021).

[3] Ge, Y., et al., in preparation.   

Electrically controlled quantum transition to an anomalous metal at LaVO3/SrTiO3 interfaces

Superconductivity (SC) in low-dimensional, low-carrier density systems is fascinating due to deviation of electron pairing mechanism from conventional BCS-Eliashberg paradigm. Insight can be gained into mechanism of emergence of SC in such systems by studying the destruction of SC through controlled disordering. We have shown that an array of well segregated superconducting islands, embedded in a semiconducting (bad metallic) matrix, can be reproducibly constructed and controllably tuned in-situ by an electric field at LaVO₃/SrTiO₃ interfaces. By controlling an electric field V_G (hence disorder), a quantum phase transition from a superconducting phase to a strange quantum anomalous metallic (QAM) phase is accomplished. In the QAM phase, the resistivity drops below a critical temperature and then saturates, indicating possibility of emergence of a Bose metal like phase. The unprecedented control over the distribution of nanometer-scale superconducting islands is obtained through the control of nanometer-scale ferroelectric domains formed in the SrTiO₃ side of the interface due to a low-temperature structural phase transition. This opens a new avenue to realize novel quantum phases in low-dimensional materials through in-situ domain engineering.   

Quasi-two-dimensional anisotropic superconductivity in Li-intercalated 2H-TaS2

Superconductivity in two-dimensional (2D) materials has sparked great interest due to the emergence of various novel quantum phenomena. Recent experimental studies of 2D superconductivity in anisotropic layered materials propose both conventional and unconventional electron pairing. In this series, layered transition metal dichalcogenides (TMDs) are an intriguing class of materials which are the potential candidates to realise Ising superconductivity, spin-valley coupling, quantum spin hall effect, and non-trivial topologically protected band structure. Chemical doping or intercalation in such materials provides a suitable way to tune the interlayer coupling and thereby tune dimensionality. 

         In this work, we have investigated the enhanced superconductivity in 2H-TaS₂ by Li intercalation and demonstrated modified superconducting properties of parent compound 2H-TaS₂ after intercalation. The magnetisation, resistivity, and specific heat measurement of single crystals confirmed weakly coupled bulk anisotropic superconductivity having transition temperature at T_c ~ 3.3(1) K. Moreover, the cusp-like feature in angle-dependent magnetotransport measurement and the appearance of Berezinskii-Kosterlitz-Thouless (BKT) phase transition in bulk crystal Li_xTaS₂ suggested the quasi-2D nature of superconductivity. These findings suggest the weakening of interlayer coupling by Li intercalation. Moreover, the readily cleavable nature of 2H- Li_xTaS₂ promotes the thickness-dependent measurement and opens a door towards the more pronounced two-dimensional nature with enhanced spin-orbit coupling. There are very few studies in intercalated materials where BKT transition is observed, which makes Li_xTaS₂ a valuable candidate for understanding the superconductivity with BKT mechanism and providing a new insight towards the intercalated materials.   

Theory of Ising superconductivity in bulk misfit heterostructures

It has recently become clear the notion that Ising superconductivity can only occur in two dimensional materials is incomplete.  A key signature of Ising superconductivity is their resilience to in-plane magnetic fields that greatly exceeds the Pauli limit.  Several experiments have shown in-plane thermodynamic critical fields in a class of bulk materials known as misfit layer compounds that greatly exceeds the Pauli limit.  We use first-principles calculations to show the combination broken inversion symmetry, large charge transfer doping and the protection of Ising spin orbit coupling in the bulk misfit compounds contains the necessary ingredients to be classified as Ising superconductors.  The results of these calculations will be discussed in the context of experiments conducted on LaSe/NbSe₂ bulk misfit heterostructures where signatures of Ising superconductivity have been reported.   

Microdiffraction Study of Superelastic CaFe2As2 micropillars.

Applied strain has become a critical tool in the study of complex materials. High levels of elastic strain can be achieved by applying uniaxial stress to micropillars. Due to the mechanical size effect, typical materials can be elastically strained thirty to forty times more as micropillars than in the bulk. In a few cases the effect can be an order of magnitude greater than that, dubbed superelasticity.¹ One material of this kind is CaFe₂As₂, a parent material for the high T_C superconductivity. Superelasticity in CaFe₂As₂ is associated with a well-known tetragonal to collapsed tetragonal phase transition that occurs under compressive stress. The transition under strain control occurs over a range of strain with DFT calculations indicating that the transition is locally abrupt but spatially dispersed forming (001) oriented phase boundaries.¹ Measuring this transition is the subject of our study. In addition to some microstructural changes, we found that above a critical level of strain a second (002) peak appears corresponding regions of the crystals with a second, shorter c-axis length. This sudden change is consistent with the proposed spatially dispersed transition and allows us to map the distribution of the two phases.

1. John T. Sypek et al., Nature Communications 8. 1083 (2017).   

Nitrogen Vacancy Centers as a Source of Solvated Charges Across the Diamond-Water Interface

Injection of charges from solid state materials into water is a poorly understood field due to the complexity of the surace chemistry, ultrafast time scales for charge injection and instability of the solvated charges. Diamond, being chemically inert in aqueous conditions, has been shown to generate solvated electrons with ultraviolet light illumination and subsequent chemical transformations have been confirmed. Now there is strong evidence that near-surface nitrogen vacancy (NV) centers in diamond can inject both electrons and holes into ultrapure water at the diamond-water interface through a process of photoionization and charge migration. Oxygen-terminated diamond members in a spectroelectrochemical cell configuration allows photocurrents to be collected as a function of bias voltage and green laser excitation. Photocurrent can be modulated by laser power and bias voltage and there is evidence that wavelength-dependent characteristics support a charge cycling mechanism with the NV center. Discovery that the NV center is a viable source of solvated electrons  expands NV diamond as a substrate for chemical reactions. Theoretical calculations also elucidate the mechanism of charge injection/solvation and reinforce the experimental observations. These results lays the ground work for future investigations that demonstrate in-situ carrier sensing across the diamond-water interface using the NV center's quantum sensing abilities.   

Towards an alternative approach to achieve superconducting infinite-layer nickelate thin films

The recent discovery of superconductivity in the family of hole-doped infinite-layer nickelates (Nd_1-xSr_xNiO₂, x=dopant concentration) has given rise to an entirely new realm of research [1]. The superconducting phase is obtained after selectively removing all the apical oxygens from the perovskite phase by means of a complex chemical topotactic reduction using CaH₂ as a reducing agent [2]. This method prone to irreproducibility issues, which is hindering the development of the field. However, very recently, Wei et al. [3] reported superconductivity in Nd_0.75Eu_0.25NiO₂ infinite-layer phase by using an Al layer as an oxygen scavenger.

In this work, we will present the whole optimization process of Pr_0.80Sr_0.20NiO₃ thin films grown on STO (001) substrates to obtain the infinite-layer phase of nickelates by using Al sputtering deposition. In addition, we will discuss the key parameters for stabilizing the perovskite phase and the infinite-layer phase, comparing samples reduced by the two methods. Our findings could help in the development of a more reliable and reproducible method to attain superconducting infinite-layer samples.

References:

[1] D. Li et al., Nature 572 (2019) 624.

[2] K. Lee et al., APL Materials 8 (2020) 041107.

[3] W. Wei et al., Science Advances 9 (27) (2023) eadh3327.   

Exotic phase transitions in spin ladders with discrete symmetries that emulate spin-1/2 bosons in 2D

We introduce a spin ladder with a set of discrete symmetries that is designed to emulate a two-dimensional spin-1/2 boson system at half-filling. Based on global properties such as the structure of topological defects, we establish the correspondence between the two systems and construct a dictionary of symmetries and operators. In particular, translation invariance leads to Lieb-Schultz-Mattis constraints for both systems, resulting in exotic deconfined quantum critical points. Subsequently, we study the spin ladder in detail. An exact duality transformation maps it onto a Z2 gauge theory of three partons, analogous to the U(1) gauge theory of chargons and spinons in two-dimensional spin-1/2 boson systems. With the mapping between spins and partons, we construct exactly solvable models for all pertinent symmetry-breaking phases and analyze their transitions. We complement the exact analysis with numerics and make connections to conventional parton gauge theories.   

Emergence of Weyl Fermions by Ferrimagnetism in a Noncentrosymmetric Magnetic Weyl Semimetal

Searching for angle-resolved photoemission spectroscopy (ARPES) signatures across magnetic phase transition is typically a difficult task, partly due to the formation of magnetic domains that are smaller than the typical beam spot size. In relation to the magnetic Weyl semimetal, many attempts have been made to study the shift of Weyl nodes across the magnetic transition. However, this is intrinsically difficult due to the smallness of the Zeeman energy. We have come up with a novel attempt, that is to look for evidence of Brillouin zone folding in a ferrimagnet. Here, using ARPES, we provide a new example of this by visualizing the electronic structure of a noncentrosymmetric magnetic Weyl semimetal candidate NdAlSi in both the paramagnetic and ferrimagnetic states. We observe surface Fermi arcs and bulk Weyl fermion dispersion as well as the regulation of Weyl fermions by ferrimagnetism. Our results establish NdAlSi as a magnetic Weyl semimetal and provide the first experimental observation of ferrimagnetic regulation of Weyl fermions in condensed matter.   

Scanning SQUID imaging of metastable states in 1T-TaS2

Visualizing the current distribution in materials is a powerful tool to investigate and understand unconventional transport they exhibit. In the present work, we study a few microns thick devices of the layered chalcogenide material 1T-TaS₂. Pulsed DC excitation of the commensurate charge density wave (CCDW) phase in the system leads to a controllable, non-volatile, resistance-switching states. We use scanning SQUID microscopy to image, in-situ, the local current density map by mapping the field generated by the current flow. The images reveal the presence of electrical domains in the device and their effect on the current flow.   

Identifying Quantum Phase Transitions with Minimal Empirical Knowledge by Machine Learning

In this work, we proposed a novel approach for identifying quantum phase transitions in one-dimensional quantum many-body systems using Autoencoders (AE), an unsupervised machine learning technique, with minimal empirical knowledge. The training of the AEs is done with data across entire range of the driving parameter and thus no prior knowledge of the phase diagram is required. With this method, we successfully detect the phase transitions in a wide range of models with multiple phase transitions of different types, including the topological and the Berezinskii-Kosterlitz-Thouless ones by tracking the changes in the reconstruction loss of the AE. The learned representation of the AE has potential utility in elucidating the physical phenomena underlying different quantum phases. Our methodology demonstrates an effective and promising new approach to studying quantum phase transitions.   

Photo-induced nonequilibrium response in underdoped cuprates probed by time-resolved terahertz spectroscopy

Utilizing ultrashort laser pulses to explore and manipulate quantum materials has led to many remarkable discoveries, one of which is the light-induced transient superconductivity. In cuprates, the transient superconductivity was first claimed in LESCO, then YBCO and LBCO through time-resolved terahertz measurements along the c-axis, though the observations varied in terms of pump fluence, polarization, wavelength, or time delay after excitation. To gain further insight into this issue, we conducted non-equilibrium terahertz spectroscopy measurements on underdoped YBCO and LBCO with tunable pump wavelengths from near-infrared to mid-infrared, in both the ab-plane and c-axis. Our results raise questions about the notion of photoinduced superconductivity and whether the observed phenomena can be exclusively interpreted as "transient superconductivity," as well as whether phonon resonant pumping is necessary to observe the phenomenon. Notably, we have found that the pump-induced superconducting condensate is absent in the ab-plane response, suggesting that the c-axis transient responses cannot be explained by an equivalent of Josephson tunneling, but instead are likely linked to the generation of quasi-particles.
Work done with S. J. Zhang, T. Dong, X. Y. Zhou, Z. X. Wang, S. X. Xu, Q. Wu, L. Yue, Q. M. Liu, T. C. Hu, R. S. Li, J. Y. Yuan, C. C. Homes, G. D. Gu.   

Universal sublinear resistivity in vanadium kagome materials hosting charge density waves

The recent discovery of a charge density (CDW) state in ScV₆Sn₆ at T_CDW = 91 K offers new opportunities to understand the origins of electronic instabilities in topological kagome systems. By comparing to the isostructural non-CDW compound LuV₆Sn₆, we unravel interesting electrical transport properties in ScV₆Sn₆, above and below T_CDW. We observed that by applying a magnetic field along the a axis, the temperature behavior of the longitudinal resistivity in ScV₆Sn₆ changes from metal-like to insulator-like above the CDW transition. We show that in the charge ordered state ScV₆Sn₆ follows the Fermi liquid behavior while above that, it transforms into a non-Fermi liquid phase in which the resistivity varies sublinearly over a broad temperature range. The sublinear resistivity, which scales by T^3/5 is a common feature among other vanadium-containing kagome compounds exhibiting CDW states such as KV₃Sb₅, RbV₃Sb₅, and CsV₃Sb₅. By contrast, the non-Fermi liquid behavior does not occur in LuV6Sn6. We explain the T^3/5 universal scaling behavior from the Coulomb scattering between Dirac electrons and Van Hove singularities; common features in the electronic structure of kagome materials. Finally, we show anomalous Hall-like behavior in ScV₆Sn₆ below T_CDW, which is absent in the Lu compound. Comparing the transport properties of ScV₆Sn₆ and LuV₆Sn₆ is valuable to highlight the impacts of the unusual CDW in the Sc compound.   

Information localization in defective spin-chain

We study Out of time order Correlators (OTOCs) of ZY 1D spin compass model[1] which is also widely known to be the 1D analogue of 2D Kitaev spin liquid model(KSL). Among the many platforms for realizing 2D KSL on trivalent graphs like hexagon or square-octagon, our recent work[2] has shown that Cyclooctatraene(COT) based polymers form a natural candidate to realizing square-octagon lattices both in 1D & 2D with interesting topological & flat-band properties. In our current work, we consider Bond-twist/flip disorders on the 1D spin compass model wherein the vicinity of the defective site, the bonds are repeated while they preserve the alternating nature of bond structure of ZZ and YY bonds before and after the defect site. Given this defective spin chain model, We observe that the XX-OTOC: both real & imaginary[3] parts, propogate only between the start of the chain upto the defective site under open boundary conditions. The absence of OTOC signal and information scrambling beyond the defect suggests the phenomena of localization of information (LOI) in the OTOC space thus enabling us to propose OTOC as a probe to detect the aforementioned defects. Additionally, the relation between LOI & Many-Body Localized(MBL) state in this model has been explored.    

Nonlinear Valley Hall Effect

The energy minima of the electronic bands in the momentum space is referred to as valley. It is now considered a new degree of freedom which has led to the emergence of the field of valleytronics. 

The linear valley Hall effect (VHE), introduced by Di Xiao et al. enables electrical control and manipulation of the valley degree of freedom. VHE is the accumulation of electrons with opposite valley indices on opposite sides of a sample, transverse to the direction of the applied electric field. It is induced by the valley contrasting Berry curvature in materials with broken inversion symmetry.  However, in systems with inversion and time-reversal symmetry, the Berry curvature and the consequent VHE vanishes. This raises a fundamental question. Can we probe the valley degree of freedom in nonmagnetic and inversion symmetric materials? If so, then how can this be achieved?

In my talk, I will delve into these questions and introduce the nonlinear valley Hall effect (NVHE) that can probe the valley degree of freedom in inversion and time-reversal symmetric systems. I will explain the underlying physical picture of the phenomena and discuss how the NVHE can be used to investigate the quantum metric in systems with both fundamental symmetries. I will discuss its connection to the electric field-induced orbital magnetic moment, and comment on the experimental aspects of NVHE. Finally, I will discuss the intrinsic and extrinsic nature of the phenomena by considering specific examples of strained graphene.   

Towards ideal band insulation: Modulating the localization tensor using optimal auxiliary fields

When applied to band insulators, electric fields cause transitions between (filled) valence bands and (empty) conduction bands. This gives rise to macroscopic electron transport. Such transport properties are closely related to the localization tensor [reviewed, e.g., in R. Resta, J. Phys.: Condens. Matter 22, 123201 (2010)], which diverges for metals but remains finite (though nonvanishing) for insulators in the thermodynamic limit. By invoking ideas related to Berry's transitionless quantum driving [M. V. Berry, J. Phys. A: Math. Theor. 42, 365303 (2009)], we seek auxiliary fields that aim to reduce the localization tensor and thus improve the quality of the insulator. By considering certain classes of auxiliary fields (e.g., ones that are feasible to produce experimentally), we optimize a certain figure of merit that directly characterizes the effects of transitions. For given classes of auxiliary fields, this strategy yields fields that optimally reduce the localization tensor, taking it as close as possible to the case of an ideal insulator, for which it would vanish.   

Heterojunctions of 2D Materials for Molecular Electronics

Molecular electronics offer an appealing alternative for future electronic devices, providing advanced functionalities that surpass the current scaling limits of silicon-based electronics. Our work demonstrates that two-dimensional (2D) materials such as graphene that come into contact with the molecular layer contribute to the formation of highly efficient devices. The fabrication of the molecular layer with Langmuir-Blodgett film was utilized to show the tunneling effect of the molecular transistor. The sharp decrease in current with increasing voltage shows a negative differential resistance effect with a high on-off ratio.

Furthermore, our research extends to the examination of thermal and thermoelectric properties within molecular junctions by incorporating molecular monolayers into a thin film configuration. Specifically, our objective is to enhance the performance of thermoelectric devices by integrating graphene-Langmuir Blodgett (LB) films.Our research also delves into Langmuir Blodgett films featuring 2D material heterojunctions, especially in the context of adaptability to diverse light sources, which is a critical aspect of our research.   

Intrinsic Magnetic Topological Insulators of the MnBi2Te4 Family: Current Status

Recently, MnBi₂Te₄ has been theoretically predicted and then experimentally confirmed to be the first intrinsic antiferromagnetic TI (AFMTI) [1-3]. This opens a new field that focuses on intrinsically magnetic stoichiometric compounds: several MnBi₂Te₄-derived MTIs were synthesized right away [4], such as (MnBi₂Te₄)·n(Bi₂Te₃), MnBi_2−xSb_xTe₄, (MnSb₂Te₄)·n(Sb₂Te₃), Mn₂(Bi,Sb)₂Te₅, and MnBi₂Se₄, that will also be discussed in the talk. As a result, MnBi₂Te₄ has been predicted to be a platform for realizing high-order topological insulator and superconductor states, Weyl semimetal phase, skyrmions, quantized magnetoeletric coupling, and Majorana fermions. Moreover, MnBi₂Te₄-based systems are predicted and/or observed to show 12 different types of Hall effect [5,6], some of them are fundamentally new, such as the layer Hall effect [6]. In MnBi₂Te₄/hBN van der Waals heterostructures, a stack of n MnBi₂Te₄ films with C = 1 intercalated by hBN monolayers gives rise to a high Chern number state, characterized by C = n chiral edge modes [7], this number being as large as allowed by the van der Waals heterostructures growth technology. Concerning current challenges of this field, we will discuss the issue of the Dirac point gap in the MnBi₂Te₄ topological surface state that caused a lot of controversy. While the early experimental measurements reported on large Dirac point gaps, in agreement with ab initio calculations, a number of further studies claimed to observe a gapless dispersion of the MnBi₂Te₄ Dirac cone. A number of possible theoretical explanations of this unexpected behavior have been put forward, which we will discuss in the context of the available experimental data [8].

 

References

[1] M.M. Otrokov et al. Nature 576 16 (2019)

[2] M.M. Otrokov et al. Phys. Rev. Lett. 122 07202 (2019)

[3] Y. Gong et al., Chin. Phys. Lett. 36, 076801 (2019)

[4] I.I. Klimovskikh et al. npj Quantum Mater. 5 54 (2020)

[5] Y. Deng et al. Science 367 (2020) 895

[6] A. Gao et al. Nature 595 (2021) 521

[7] M. Bosnar et al. npj 2D Mater. Appl. 7 33 (2023)

[8] M. Garnica et al. npj Quantum Mater. 7 7 (2022)   

Observation of single-element ferroelectricity in a two-dimensional bismuth layer

Ferroelectrics are intriguing due to their non-volatile spontaneous polarization, which has applications in data storage, electric sensors, and even photovoltaic cells and neuromorphic devices. Since polarization arise exclusively from ordered anions and cations, making ferroelectric materials compound in nature. Although some theoretical works have attempted to expand and enhance our comprehension of ferroelectricity by challenging the conventional boundary of ionic polarization in elementary substances, experimental advancement continue to be impeded by the complex characteristics of single-element materials at low dimensions.
In this work, we employed the scanning tunnelling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) to investigate the black-phosphorus-like Bi (BP-Bi) monolayer. Different from the centrosymmetric black phosphorus, the BP-Bi monolayer exhibits a buckled atomic arrangement between its two sublattice due to weak and anisotropic sp orbital hybridization, thus supporting the noncentrosymmetry in BP-Bi. Our electronic state measurements and kelvin probe force microscopy (KPFM) also reveal electron redistribution between the two sublattices, suggesting an in-plane polarization. Most importantly, by applying an in-plane electric field using STM, we successfully demonstrate the polarization switching at both microscopic and macroscopic scales. This experimental discovery unveils the emergent single-element ferroelectricity, which has the potential to expand our understanding of ferroelectric mechanism and facilitate the development of ferroelectric devices in the future.   

Oral: Strange Metallicity and Magnetic Order in the CoNi(Cr/V) Medium-Entropy Alloy System

CoNiCr is a prototypical example of topical multi-principle element alloys with superior cryogenic and high-temperature mechanical strength, corrosion, oxidation resistance, and yet-to-be-explored magnetic and electronic functionalities. The remarkable properties of this transition metal ternary system are not only due to atomic radii, electronic configurational mismatch, and atomic volume misfit but are also dependent on the debated magnetically driven chemical short-range order. The current study focuses on the electric and magnetic properties of the single-phase face-centered cubic CoNi(Cr/V) system in which V is introduced to the system at the expense of Cr to fine-tune the volume misfit in the system. All the samples exhibited ultra-small magnetic moments due to the complex magnetic interactions of the constituent elements. The electric transport measurements revealed a strange metallicity evidenced through the observation of the linear temperature dependence of the resistivity. Our findings support the recent theoretical studies on the magnetically driven chemical short-range order of the CoNiCr system.   

Gate-tuning of magnetism in a van der Waals magnetic semimetal Cr3Te4

The concept of a gate-induced modulation of a quantum state of matter has become more and more popular since the beginning of this century. In particular, a gate control of magnetism has been one of the central topics in this research field, and in fact significant achievements have been published including a recent paper on gate-tunable room temperature ferromagnetism in a van der Waals (vdW) magnetic metal, Fe₃GeTe₂ [1]. However, a material system explored so far has been limited either to magnetic semiconductors or to magnetic metals, whereas gating effects in magnetic semimetals have been largely unexplored despite the unique features of this class of materials associated with the band-crossing points in the momentum space. This peculiar band structure dominates overall magnetic properties (the Curie temperature (T_C), anisotropy, and magnetotransport properties), which underpins the attractiveness of magnetic semimetals. Because the band-structure-driven magnetism in magnetic semimetals should be extremely sensitive to the Fermi level (E_F), magnetic semimetals are one of the most attractive targets for gate control.

Here we report the gating effects in a vdW magnetic semimetal for the first time. We focus on Cr₃Te₄, an emergent vdW magnetic semimetal [2, 3], and unveil its anomalous carrier-dependent ferromagnetism through a series of the ion-gating experiments. In the presentation, we shows the detailed results on the gate-tunable magnetic properties of Cr₃Te₄, including T_C, magnetic anisotropy, and magnetotransport properties. For details of this study, see the preprint at [4].   

Quantum phase diagram of the extended Spin-3/2 Kitaev Heisenberg model on three- and four-leg cylinders

Recently there has been considerable excitement surrounding the promising realization of high-spin Kitaev material, such as the quasi-2D compound CrI₃ and CrGeTe₃. However, the stability of quantum spin liquids (QSL) against the extra single ion anisotropy (SIA) in these high-spin materials and the global quantum phase diagram of the extended Kitaev model with SIA term remains still unclear. In this work, we perform large-scale density matrix renormalization group to explore the quantum phase diagram of the generalized spin-3/2 Kitaev-Heisenberg (K-H) model accompanied with the SIA interaction A_c. In the A_c=0 limit, the spin-3/2 K-H model exhibits a similar quantum phase diagram as the one of the spin-1/2 system, including two QSLs around antiferromagnetic and ferromagnetic Kitaev models. As the SIA interaction increases, we find that the QSL in the AFM Kitaev model is fragile against the SIA interaction and a first order phase transition between QSL and AFM ordered phase happens at A_c ∼ 0.05. While for the FM Kitaev model, a new magnetic ordered phase emerges through a continuous phase transition at A_c ∼ 0.04. The effect of the SIA term on other magnetic ordered phases is also discussed.   

Vison manipulation with local magnetic fields

Quantum spin liquids hosting non-abelian anyons have recently experienced renewed interest following the discovery of a variety of materials proximate to these quantum phases. Their anyonic excitations have potential for application to topological quantum computation, but designing logical operations with reduced poisoning requires developing protocols to faithfully create, move and read-out such quasiparticles. In this paper, we present one such protocol for manipulating Z2 fluxes (``visons'') of ferromagnetic and antiferromagnetic Kitaev models perturbed by a small uniform magnetic field. Our design employs a local driving magnetic field and can achieve high probabilities of generating and displacing flux pairs of both the Az and B phases of the model.   

Spin Space Group Theory and Unconventional Magnons in Collinear Magnets

Topological magnons have received substantial interest for their potential in both fundamental research and device applications due to their exotic uncharged yet topologically protected boundary modes. However, their understanding has been impeded by the lack of fundamental symmetry descriptions of magnetic materials, of which the spin Hamiltonians are essentially determined by the isotropic Heisenberg interaction. The corresponding magnon band structures allows for more symmetry operations with separated spin and spatial operations, forming spin space groups (SSGs), than the conventional magnetic space groups. Here we developed spin space group (SSG) theory to describe collinear magnetic configurations, identifying all the 1421 collinear SSGs and categorizing them into four types, constructing band representations for these SSGs, and providing a full tabulation of SSGs with exotic nodal topology. Our representation theory perfectly explains the band degeneracies of previous experiments and identifies new magnons beyond magnetic space groups with topological charges, including duodecuple point, octuple nodal line and charge-4 octuple point. With an efficient algorithm that diagnoses topological magnons in collinear magnets, our work offers new pathways to exploring exotic phenomena of magnonic systems, with the potential to advance the next-generation spintronic devices.   

Chiral phonons and phononic birefringence in ferromagnetic metal - bulk acoustic resonator hybrids

Magnetoelastic coupling between excitation modes of the spin system (spin waves) and the lattice (phonons) is of interest from a fundamental perspective and can enable mode hybridization. For quantum sensing and transduction protocols, excitation transfer between the magnetic and elastic systems is of importance, but typically requires strong coupling between the magnetic and elastic modes. Here, we present our current results on coupling the magnetization dynamics in a ferromagnetic thin film to a high-overtone bulk acoustic resonator. We show that the typically weak coupling affects the magnetization dynamics of the magnetic layer and can thereby be characterized with high sensitivity using broadband ferromagnetic resonance (bbFMR) spectroscopy [1]. In our experiments [2], we investigate the magnetoelastic coupling of polycrystalline metallic thin films deposited on silicon and sapphire substrates via DC sputter deposition as a function of microwave excitation frequency and substrate material by performing bbFMR experiments. Utilizing a model based on coherent magnetoelastic coupling and the phononic properties of the substrate material, we obtain a full description of the observed changes in ferromagnetic resonance as a function of microwave frequency. Furthermore, we will discuss the implications of this model for the phononic angular momentum transport.

[1] R. Schlitz et al., Phys. Rev. B 106, 014407 (2022).

[2] M. Müller et al., arXiv:2303.08429 (2023).   

Design principles for ferroelectricity induced by antiferromagnetic ordering

Coupling between electrical dipole and magnetism is the key ingredient in the search for multiferroics. On the other hand, functional antiferromagnets have attracted enormous interests and significantly expand the pool of spintronic materials. With the help of recently established spin space group theory, we have filtered out the space groups that allow electrical polarization emerging from antiferromagnetic spin ordering. A new mechanism to induce the electrical polarization purely by nested antiferromagnetic sublattices is identified. Following the guidance, we proposed several candidates and computed the polarization and bulk photovoltaic effect using first-principles calculations. Our work paves the way for designing functional multiferroics with antiferromagnetic materials without spin-orbit coupling.   

Asymmetrical energy landscape for domain wall motion induced by compositional gradients in ferromagnetic nanowires

In recent years, 3D magnetic systems and curvilinear nanostructures have appeared as exciting alternatives for developing novel spintronics applications based on domain wall motion. In particular, complex and interesting spin textures and novel physics phenomena can be observed in cylindrical nanowires. Electrodeposition is a versatile tool for the synthesis of cylindrical nanowires with controlled morphology and composition. Recent works have shown that introducing local changes in composition along the axial direction in permalloy nanowires is very efficient for controlling the magnetisation dynamics. 

We will show in this work that electrochemical deposition also allows the introduction of gradual changes in the Fe/Ni ratio along the axis of the nanowires. This introduces a gradual change of the ferromagnetic properties, producing an asymmetrical energy landscape for the magnetic domain walls along the nanowire, as shown by micromagnetic simulations. In addition, these compositional gradients produce an asymmetrical landscape for domain wall motion, which is reflected in asymmetrical magnetisation processes under an applied magnetic field, paving the way towards full control of the movement of domain walls along these nanowires.   

EPR Properties of Irradiated Hydroxyapatite and Fluorapatite Cements: Carbonated and Sodium Effects

In a recent study, electron paramagnetic resonance (EPR) spectra of γ-radiation-induced paramagnetic defects in carbonated hydroxyapatite (CHA), prepared via calcium phosphate cement method, revealed a pure and stable EPR signal from orthorhombic CO₂^●– (g_x = 2.0029, g_y = 1.9967, g_z = 2.0017) radicals. The signal is proportional to the dose received from 5 Gy to 20 kGy¹. The findings suggested that CHA could be a promising substrate for EPR-based dosimetry. Understanding the effects of anion and cation incorporation on the EPR properties of apatite cements is critical to improving their dynamic range for dosimetry. Here, we studied the impact of carbonate (CO₃^2–) and sodium (Na) concentrations on the EPR properties of irradiated hydroxyapatite (HA) and Fluorapatite (FAP) cements. We synthesized nine HA and nine FAP cement samples, each containing three levels of CO₃^2– and three levels of Na. After γ-irradiation at 10 kGy, EPR spectra revealed a major EPR signal from orthorhombic CO₂^●– and a smaller signal from CO₃^●– (g_x = 2.0091, g_y = 2.01718, g_z = 2.00584) radicals. For a given pair of CO₃^2– and Na contents, FAP generally produced significantly stronger EPR signals than did HA. Without added Na, the FAP/HA EPR signal intensity ratio was 2.2 ± 0.88; this ratio decreased to 1.09 ± 1.31 as Na concentration was increased. These findings show that changing the elemental content improves the EPR signal intensity in apatite cements.   

Microscopic Mechanism of a Twisted Kitaev Model

For many years CoNb₂O₆ was believed to be described by the transverse-field Ising model (TFIM). However, recent experimental evidence and symmetry analysis of the crystal structure suggests this compound can be described by variations of the twisted Kitaev model (TKM), which features bond-dependent interactions. Here we study a microscopic mechanism to generate the twisted Kitaev exchange interaction, among others, using a strong-coupling expansion. We combine these results with tight-binding parameters obtained by density-functional theory, and use exact diagonalization to compare the magnetic spectrum of our model with inelastic neutron scattering data.   

Ising Fracton Spin Liquid on the Honeycomb Lattice

Fractons are quasiparticles which are incapable of independent motion. They arise naturally as a consequence of dipole conservation in higher-rank gauge theories. One route to experimental realization of fractonic physics is therefore to construct models realizing such exotic gauge theories, ideally built from short-ranged two-body interactions. This has been done successfully in the context of classical spin systems, however these models have been constructed from continuous degrees of freedom, making it impossible to isolate and study discrete fractons. Here, we present an Ising model exhibiting a fractonic spin liquid regime. We show explicitly that the excitations are fractons, appearing at the corners of membranes of spin flips. Because of the three-fold symmetry of the honeycomb lattice, these membranes can be locally combined such that no excitations are created, giving rise to a set of ground states described as a liquid of membranes. The liquid nature of the low-energy state sets our model apart from other known classical models that host fracton excitations. To study the finite-temperature behavior of the model, we devise a bespoke cluster Monte-Carlo algorithm, that moves pairs of defects and thus overcomes the freezing induced by the otherwise immobile excitations. We find evidence for a first order transition from a high-temperature paramagnet to a low-temperature phase whose correlations precisely match those predicted for a higher-rank Coulomb phase.   

GW-RPA studies of interacting electron gas models

In this work, we study the ground states and single-particle excitation spectra of both two dimensional (2D) and three dimensional (3D) electron-gas models with quadratic band dispersions using many-body perturbation methods. We start with unrestricted Hartree-Fock calculations that may lead to either Wigner crystal or Fermi liquid state at low carrier densities, and we further calculate the GW quasi-particle energies based on the two different types of ground states. The correlation energy is treated with random phase approximation (RPA) based on the GW quasi-particle energies. We apply such Hartree-Fock+GW+RPA method to study the Wigner crystal transitions in both 2D and 3D interacting electron-gas models, and demonstrate that the critical Wigner-Seitz radii can be more accurately determined compared to the bare Hartree-Fock methods.   

Machine-Learning-Based Non-Local Kinetic Energy Density Functional for Simple Metals and Alloys

The development of an accurate kinetic energy density functional (KEDF) remains a major hurdle in orbital-free density functional theory (OFDFT). By constructing a series of truncated non-local KEDFs, we find that considering the interactions between an atom and its next nearest neighbor atoms is crucial for a non-local KEDF to distinguish the energy orderings among bulk structures, as well as to accurately predict the surface energies and point vacancy energies of Al and Si systems. Furthermore, we propose a machine-learning-based non-local KEDF, which is subject to three exact physical constraints: the scaling law of electron kinetic energy, the free electron gas limit, and the non-negativity of Pauli energy density. The machine-learning-based KEDF is systematically tested for simple metals, including Li, Mg, Al, and 59 alloys. We conclude that incorporating non-local information for designing new KEDFs and obeying exact physical constraints are essential to improve the accuracy, transferability, and stability of ML-based KEDF. These results shed new light on the construction of KEDFs.   

Deconvoluting the Electric-Shielding Effect of Ubiquitous Water in Kelvin Probe Force Microscopy: Finite Element Approach

An electrostatic Finite Element (FE) model was employed to investigate how the presence of a water meniscus influences the surface potential difference between a simulated tip at a constant electric potential and a flat surface. This numerical approach provided valuable insights into the interplay between a two-dimensional vertical heterostructure material surface and ambient water, which holds importance in the design of functional devices. Kelvin Probe Force Microscopy (KPFM), an experimental technique used to study electronic and topological properties of material surfaces with nanoscale precision, was conducted in varying relative humidity environments. Our numerical methodology and experimental KPFM findings exhibited a close correspondence, revealing an inverse relationship between rising humidity levels and the reduced contrast in surface potential. Based on our results, we posit that this extrinsic effect can be attributed to the electric potential shielding effect of water and therefore subtracted in the experimental KPFM to understand the intrinsic properties of a sample in the presence of ambient water.   

Finite-bias molecular dynamics simulations of water at the electrified graphene surface

A fundamental understanding of the electrified electrochemical interfaces in atomic scale holds critical implications for the development of advanced energy storage and conversion devices. For this purpose, first-principles characterizations based on the approach combining density functional theory (DFT) and non-equilibrium Green’s function (NEGF) have been utilized with much successes. However, due to the requirement of semi-infinite electrodes, the DFT-NEGF approach so far could not be adopted to graphene-based electrified electrochemical interface models. In this presentation, taking the advantage of the multi-space constrained-search DFT (MS-DFT) formalism [1-3] that can handle the electrified two-dimensional electrodes, we firstly investigate the non-equilibrium energetics of water clusters connected through hydrogen bond network on the electrified graphene electrode surface. Next, by carrying out non-equilibrium molecular dynamics (MD) simulations based on MS-DFT, we study the bias-dependent configurations of few-layer water on the electrified graphene interfaces. Drastic difference between the interfacial water properties on graphene electrode with those on normal metal electrodes will be revealed.

[1] J. Lee et al. Proc. Natl. Acad. Sci. U.S.A., 117, 10142 (2020)

[2] J. Lee et al. Adv. Sci., 7, 2001038 (2020)

[3] T.H. Kim et al., Npj Comput. Mater. 8, 50 (2022)   

Hydrodynamic Heat Transport Modeling: Continuum Models and the BTE

The latest developments in microelectronics working at the nanometer scale, along with advancements in producing ultrapure monocrystalline materials and the discovery of the existence of second sound at 200 K in graphite, as reported by Ding et al. (Nature Communications 2021), indicates importance of hydrodynamic heat transport in technology. On a device scale in complex geometries, a direct solution of the Boltzmann Transport Equation may not be computationally feasible. Consequently, there is a need for models suitable for efficient computational methods, such as the Finite Element Method.

Early attempts to bypass the difficulty of directly solving the BTE included the development of the Guyer and Krumhansl (GK) equation. More recently, coarse-graining approaches to the BTE have been used to formulate the partial-differential Viscous Heat Equations (VHEs), and to predict the emergence of intriguing effects such as heat backflow and non-diffusive temperature profiles (Dragašević and Simoncelli, arXiv 2023). This work takes the next step in analyzing the VHEs and proposes an exact analytical solution for the Transient Thermal Grating geometry, allowing for a direct comparison with the recently developed semi-analytical approach to solving the full scattering matrix BTE. Furthermore, the BTE, VHEs, and GK equations are employed to solve different initial-boundary value problems, and their accuracies and regimes of applicability are systematically compared.   

Quantum Information Science with Reconfigurable Photonic Waveguide Array

Photonic waveguide array has been used in a number of applications ranging from demonstrating condensed matter physics and processing classical and quantum

information. However, those devices lack reconfigurability or precise control of the device Hamiltonian. In this study, we present a Reconfigurable Waveguide Array (RWA) with 11 waveguides based on Lithium Niobate and experimentally demonstrate various applications including programming Hamiltonians, tunable quantum interference, and multi-dimensional quantum logics. These results open up new avenues for exploring the potential of this technology in photonic quantum computing.   

The feed-forward latency requirements for Quantum Error Correction

One of the greatest challenges in performing fault-tolerant quantum computing in quantum error correction (QEC) is reaching low feed-forward latencies, that is, a short time from the physical execution of a logical measurement until the controller plays a conditional pulse which depends on the logical measurement outcome. The necessity for feed-forward arises from the requirement to perform non-Clifford gates to reach quantum advantage (Gottesman-Knill theorem). To keep track of the logical flips and correct them without propagation, the conditional feed-forward of each non-Clifford gate must depend on the decoding. Here, we provide a general analysis of the feed-forward latency requirements with the latency behavior in different classical setups. Using a dynamical system analysis we show the conditions on the system latency performance that determine the operation regime: latency divergence, where quantum calculations are unfeasible, classical-controller limited runtime, or quantum-operation limited runtime where the classical operations do not delay the quantum circuit. The proposed analysis can be used for any decoding algorithm and any QEC stabilizer code towards fault-tolerant quantum computation.   

Control Requirements and Benchmarks for Quantum Error Correction

Reaching fault-tolerant quantum computation depends on the successful implementation of quantum error correction (QEC). In QEC, quantum gates and measurements are performed to stabilize the computational qubits, while classical computations convert the measurements into estimated logical Pauli frame updates or logical measurement results. While QEC research has concentrated on developing and evaluating QEC codes and decoding algorithms, specification and clarification of the requirements for the classical control system running QEC codes are lacking. Here, we elucidate the roles of the QEC control system, the necessity to implement low latency and parallelizable feed-forward quantum operations, and suggest near-term benchmarks that confront the classical bottlenecks for QEC quantum computation. These benchmarks are based on the latency between a measurement and the operation that depends on it, and incorporate the different control aspects such as quantum-classical parallelization capabilities and decoding throughput. The proposed benchmarks aim to allow the evaluation and development of scalable building blocks of QEC control system toward its realization as a main component in fault-tolerant quantum computing.   

Decohering tensor network quantum machine learning models

Tensor network quantum machine learning (QML) models are promising applications on near-term quantum hardware. While decoherence of qubits is expected to decrease the performance of QML models, it is unclear to what extent the diminished performance can be compensated for by adding ancillas to the models and accordingly increasing the virtual bond dimension of the models. Can an increased bond dimension fully compensates for the decoherence of the network, and shed light on the role of quantum coherence in QML? We investigate here the competition between decoherence and adding ancillas on the classification performance of two models, with an analysis of the decoherence effect from the perspective of regression. We present numerical evidence that the fully-decohered unitary tree tensor network (TTN) with two ancillas performs at least as well as the non-decohered unitary TTN, suggesting that it is beneficial to add at least two ancillas to the unitary TTN regardless of the amount of decoherence may be consequently introduced.   

Experimental graybox quantum identification and control

Modeling and controlling a quantum system are crucial for developing useful quantum devices to overcome errors that come from environmental noise or fabrication imperfections. Many methods have been developed to tackle this challenge. However, they are either limited by the system model such as traditional curve fitting, or lack of physics insights such as machine learning methods. Here we experimentally demonstrate a 'graybox' method on a reconfigurable photonic circuit. Our results show that the 'graybox' method outperforms the traditional model fitting method while holding the capability of providing physics insights. This new method is effective in modeling devices whose properties cannot be measured directly and can be applied to time-dependent and open quantum systems.   

Energy-dependent barren plateau in bosonic variational quantum circuits.

Bosonic continuous-variable Variational quantum circuits (VQCs) are crucial for information processing in cavity quantum electrodynamics and optical systems, widely applicable in quantum communication, sensing and error correction. The trainability of such VQCs is less understood, hindered by the lack of theoretical tools such as t-design due to the infinite dimension of the physical systems involved. We overcome this difficulty to reveal an energy-dependent barren plateau in such VQCs. The variance of the gradient decays as 1/E^Mν, exponential in the number of modes M but polynomial in the (per-mode) circuit energy E. The exponent ν = 1 for shallow circuits and ν = 2 for deep circuits. We prove these results for state preparation of general Gaussian states and number states. We also provide numerical evidence that the results extend to general state preparation tasks. As circuit energy is a controllable parameter, we provide a strategy to mitigate the barren plateau in continuous-variable VQCs.   

Optomechanical resource for fault-tolerant quantum computing

Fusion-based quantum computing with dual-rail qubits is the leading candidate for scalable quantum computing using linear optics. This computing paradigm requires single photons which are entangled into small resource states before being fed into a fusion network. The most popular sources for single optical photons (e.g., spontaneous parametric down-conversion) and for small entangled states (e.g., linear optics) are probabilistic and heralded. It is possible to effect deterministic sources of these photonic resources from many probabilistic, heralded resource generators, but this requires complex optical networks. Alternatively, successfully generated resources can be stored in quantum memories to be retrieved as needed. In this work, we show how optomechanics can be harnessed to implement such quantum memories. The acoustic modes act as caches of quantum resources—single-particle states and even small entangled states—with on-demand read-out. I will show how the resource states can be prepared directly in the acoustic modes using optical controls. This will still be probabilistic and heralded, but the acoustic modes store the quantum states with no extra effort. The quantum states may be transferred from acoustic modes to optical modes, as needed, with another optical drive. The advantages of acoustic modes as optical quantum memories, compared to other technologies, include their intrinsically long lifetimes, as well as being solid state, highly tailorable, and insensitive to electronic or magnetic noise.   

Modelling of flux-pumped Josephson Parametric Amplifiers with auxiliary circuits

Flux-pumped SQUID-based resonators can serve as parametric amplifiers. If the SQUID-based resonator is coupled to a frequency-independent environment such as a transmission line (TL), the bandwidth is limited. Inserting an auxiliary circuit into the transmission line can make the environment frequency-dependent and modify the susceptibility matrix by changing the self-energy. We show how the insertion of an auxiliary circuit affects the self-energy and thereby the susceptibility, thus facilitating the engineering of the amplifier bandwidth. We consider not only a classical model but also a quantum model, where we diagonalize the TL Hamiltonian by solving the TL normal modes subject to a boundary condition from the rest of the circuit. We also relate the self-energy to the environment impedance. We specifically discuss bandwidth engineering in the cases of in-series LC and parallel LC circuits as coupling circuits. In these cases the TL eigenmodes can be calculated, thus enabling us to find the exact Hamiltonian and coupling coefficients of the coupled system. Our results could also be useful for studying the influence of auxiliary circuits for other amplifiers.   

Multiplexed control scheme for scalable superconducting quantum processor

In this research, we present a multiplexed control scheme compatible with state-of-the-art superconducting qubit technologies, addressing critical challenges in large-scale quantum information processing. Our design allows simultaneous precise control of multiple qubits and couplers through shared control lines, markedly reducing wiring complexity, cooling power and space requirements. We also provide variant solutions according the gate type and qubit type. We expect the demand for control lines can be reduced by 1-2 orders of magnitude in the near future, presenting a promising pathway for scaling up quantum processors.   

Statistical Mechanics of Quantum Codes

The many-body dynamics of quantum circuits and quantum error correction are a rapidly developing application of computational statistical mechanics. Phenomena such as the growth of entanglement, robust encoding of quantum information, and successful decoding under noise all lend themselves to statistical mechanics mappings. In this talk, I will highlight some paradigmatic quantum coding phases and phase transitions, along with the computational methods used to study them. I will further discuss a sampling of our ongoing work in this field, which leverages the classical simulatability of Clifford circuits to probe large system sizes. A common thread in this research is “derandomization," evolving a state under a spacetime translation-invariant dynamics. Lack of randomness can reduce the computational complexity of encoding and decoding, while sometimes modifying the universality class of the associated transitions. These transitions include error-correction thresholds in dynamically generated codes; unbinding of entanglement membranes due to measurements and/or dissipation; and fault-tolerant thresholds in codes prepared by multitree circuits. For the latter, tensor network methods for evaluating and updating marginal probabilities underlie the approximate “probability passing” decoding algorithm for realistic noise models.   

Deterministic generation of hybrid entangled states using quantum walks

Recently, hybrid entanglement (HE), which involves entangling a qubit with a coherent state, has demonstrated superior performance in various quantum information processing tasks, particularly in quantum key distribution [arXiv:2305.18906 (2023)]. Despite its theoretical advantages, the practical generation of these states in the laboratory has been a challenge.

In this context, we introduce a deterministic and efficient approach for generating HE states using quantum walks. Our method achieves a remarkable fidelity of 99.90 % with just 20 time steps in a one-dimensional split-step quantum walk. This represents a significant improvement over prior approaches that yielded HE states only probabilistically, often with fidelities as low as 80%. Our scheme not only provides a robust solution to the generation of HE states but also highlights a unique advantage of quantum walks, thereby contributing to the advancement of this burgeoning field. Moreover, our scheme is experimentally feasible with the current technology.   

Analyzing simulation of quantum systems in Schrödinger and Heisenberg pictures.

Computing operator expectation values is a central task in simulating quantum many-body systems. For unitarily prepared trial states, one could either compute expectation values in the Schrodinger picture (state evolution) or Heisenberg picture (operator evolution). On one hand, full knowledge of the wavefunction permits the computation of any operator, it might not be necessary to calculate certain expectation values. Physical Hamiltonians, on the other hand, are generally sparse, and only involve a polynomial number of operators. While the space of operators is much larger than that of states, there are cases where the sparsity of the operator evolution is greater than the sparsity of the state evolution. In this talk, we compare the two approaches for a few relevant examples, including molecular simulation and the recent 127 qubit IBM experimental simulation of kicked Ising dynamics.   

Theory of valley physics in SiGe quantum dots

The weak spin-orbit coupling and the nuclear zero-spin isotopes of silicon and germanium make Si/Ge quantum dots an ideal host for semiconductor spin qubits. However, the degeneracy of the conduction band minima of bulk silicon, known as valleys, limits the performance and scalability of quantum information processing, because the valley degree of freedom competes with the spin as a low-energy two-level system. The valley degeneracy is lifted in quantum dots in Si/SiGe heterostructures due to biaxial strain and a sharp interface potential, but the reported valley splittings are often uncontrolled and can be as low as 10 to 100 μeV. This presentation will discuss in detail the main challenges for the enhancement and control of the valley splitting in silicon quantum dots. In addition, it will describe a new three-dimensional model within the effective mass theory for the calculation of the valley splitting in Si/SiGe heterostructures, which takes into account concentration fluctuations at the interface and the lateral confinement. With this model, we predicted the valley splitting as a function of various parameters, such as, the width of the interface, the electric field and the size and location of the quantum dot.   

Reduced Routing Cost in Compiling Quantum Programs with Transformers and Tree Search

The success of quantum computing relies on an effective usage of the underlying Quantum Processing Units (QPUs). Compiling a logical quantum circuit tailored to the specific architecture of a physical QPU is a necessary step to run quantum circuits. This research presents a novel approach to quantum circuit compilation using Reinforcement Learning (RL). While traditional methods using solvers or heuristics produce correct compilation results, they suffer from three drawbacks: (1) their search takes a long time to run, (2) heuristics produce sub-par results, and (3) they require domain expertise to redesign to tailor to different QPUs.

Our RL-based model automatically generates compilation “experiences” to learn from to find optimized compilation policies. The RL-based method improves the three shortcomings of traditional methods. First, an RL agent simply does model inference during compilation with a significantly faster runtime. Second, an RL agent automatically learns a good strategy given arbitrary QPU architectures and target benchmarks to provide better performance. Third, RL replaces domain expertise development time by automatic model training time, hence eliminating a huge bottleneck in quantum system design.   

Strong tunable coupling between two distant superconducting spin qubits

Superconducting (or Andreev) spin qubits have recently emerged as an alternative qubit platform with realizations in semiconductor-superconductor hybrid nanowires. In these qubits, the spin degree of freedom is intrinsically coupled to the supercurrent across a Josephson junction via the spin-orbit interaction, which facilitates fast, high-fidelity spin readout using circuit quantum electrodynamics techniques. Moreover, this spin-supercurrent coupling has been predicted to facilitate inductive multi-qubit coupling. In this work, we demonstrate a strong supercurrent-mediated coupling between two distant Andreev spin qubits. This qubit-qubit interaction is of the longitudinal type and we show that it is both gate- and flux-tunable up to a coupling strength of 178 MHz. Finally, we find that the coupling can be switched off in-situ using a magnetic flux. Our results demonstrate that integrating microscopic spin states into a superconducting qubit architecture can combine the advantages of both semiconductors and superconducting circuits and pave the way to fast two-qubit gates between remote spins.   

The quatification of the interband transitions in the quantum confined Stark effect

The potential applications in optical devices such as μ-LED, high-speed modulators, wavelength selective detectors and optically bistable switches have motivated strong interest in the quantum confined Stark effect (QCSE) in the heterostructures. However, the quantification of the QCSE is difficult and complicated which is currently limited to the allowed interband transitions. For the forbidden transitions, the solving process is time-consuming and the crossings of the forbidden transitions may appear when the electric field increases. Thus, a more accurate description of the light emission or absorption properties of quantum wells under the applied electric field is urgently needed. Here, we propose a 3D-plot to study the interband transitions. By the Wentzel–Kramers–Brillouin approximations, we deduce that the average interband transition spacing ΔE below or near the E_g is innert to the electric field strength and the ΔE is inversly proportional to the square of the quantum well's width L by 4πh_bar^2/sqrt(m_em_h) below the E_g and 12πh_bar^2/sqrt(m_em_h) slightly above the E_g . The simulations further validate the 2 scaling laws and the influence of the charge's screening is also studied.   

High-Fidelity Entangling Gates for a Register based on a Nitrogen-Vacancy Center in Diamond

Motivated by the recent experimental progress in exploring the use of a nitrogen vacancy center in diamond as a quantum computing platform, we propose schemes for fast, high-fidelity entangling gates on this platform. Using both analytical and numerical calculations, we demonstrate that synchronization effects between resonant and off-resonant transitions may be exploited such that spin-flip errors due to strong driving may be eliminated by adjusting the gate time, the driving field, or the DC magnetic field. This allows for fast, high-fidelity entangling operations between the electron spin and one or several nuclear spins. These schemes have been predicted theoretically and demonstrated experimentally for operations in semiconductor quantum dots, and we show their applicability to state-of-the-art protocols for conditionally driving either the electron spin or the nuclear spins using DDRF techniques.   

Subspace Correction for Constraints

We demonstrate that it is possible to construct operators that stabilize the constraint-satisfying subspaces of computational problems in their Ising representations. We explicitly construct concrete unitaries and associated measurements for some common constraints. The stabilizer measurements allow the detection of constraint violations, and provide a route to recovery back into the constrained subspace. We call this technique "subspace correction". As an example, we explicitly investigate the stabilizers using the simplest local constraint subspace: Independent Set. We find an algorithm that is guaranteed to produce a perfect uniform or weighted distribution over all constraint-satisfying states when paired with a stopping condition: a quantum analogue of partial rejection sampling.   

Towards quantum control of an ultracoherent mechanical resonator with a fluxonium qubit

Beyond their applications in quantum computing, superconducting qubits are a powerful platform to probe various quantum phenomena in the context of hybrid quantum systems [1].  However, most of them are confined to the GHz frequency domain, limiting the class of systems they can interact with. Building upon the heavy fluxonium architecture introduced by [2], we have developed a superconducting qubit with an unprecedentedly low transition frequency of 1.8 MHz [3]. Notably, we have demonstrated a qubit with a coherence time exceeding 30 μs, a sideband cooling scheme to prepare the qubit in a pure state with 97.7% fidelity, and single-shot readout capability. Moreover, by detecting a weak charge modulation by repeated qubit interrogation, we demonstrate the high-sensitivity of this qubit architecture to a nearly resonant AC-charge drive, proving its potential in a hybrid circuit scenario. We will finally present our recent efforts to achieve the strong coupling regime between this qubit and an ultra-coherent softly-clamped mechanical membrane.

[1] Y. Chu et al. Nature 563, 666 (2018).

[2] H. Zhang et al. Physical Review X 11, 011010 (2021).

[3] Najera et al. arXiv:2307.14329 (2023). In review at PRX.   

Dynamical Compensation for AC Stark Shift During Kerr-Cat Qubit Initial-State Preparation

A Kerr-cat qubit can be realized by applying a two-photon drive to a nonlinear resonator with small Kerr. However, such a drive shifts the frequency of the qubit, which is known as the a.c. Stark shift. Furthermore, during the ramping up process the a.c. Stark shift is dynamically increased as a function of the drive amplitude. If not compensated correctly, this a.c. Start shift will lead to high-level excitation of the Kerr-cat qubit, resulting in quantum information leakage. In this work, we provide a dynamic compensation method to improve the fidelity of the initial-state preparation of Kerr-cat qubit by slowly changing the drive frequency to dynamically ensure that at each moment the a.c. Stark shift is properly compensated. We show the significant difference between cases with and without the dynamic compensation method when the a.c. Stark shift is large. This work provides a potential approach for preparing large cat qubit and can be extended to other cat qubit operations.   

Quantum state transfer via continuous quantum teleportation

Quantum state transfer is traditionally realized either by using quantum information carriers or quantum teleportation based on classical information transmission. In this work, we propose a continuous version of the quantum teleportation protocol to transfer the quantum states between different bosonic modes in superconducting systems. The scheme relies on only two-mode squeezing interaction and avoids all excitation-exchange operations between different parties based on both quantum and classical channels. The state transfer is coherent and bidirectional and works for encoded bosonic codes for quantum error correction. Our scheme also applies to other photonic and phononic systems and helps to deepen the understanding of quantum teleportation and quantum state transfer.   

Tracking and adapting to non-stationary noise in quantum channels

Errors in quantum computing stem from various sources e.g. qubit cross-talk, environmental coupling, pulse distortion, and electromagnetic shielding issues. Non-stationary statistics however challenge reproducibility of error mitigated outcomes obtained using probabilistic error cancellation. We present results that quantify the importance of well-characterized hardware for ensuring outcome stability. We evaluate circuit performance in non-stationary conditions, comparing expected outcomes to those observed on transmon devices. We establish performance bounds based on characterization data.Our results indicate that outcome stability is bounded by the Hellinger distance between the joint distribution of noise parameters. We also explore Bayesian inference for adaptive noise estimation to enhance PEC accuracy in the Bernstein-Vazirani test circuit using decoherence data from the ibm_kolkata device. We establish a direct connection between inference efficiency and PEC accuracy in the presence of non-stationary amplitude and phase damping noise affecting CNOT gates. When Bayesian inference is 90% accurate, PEC mitigated observable error stays below 20%. If accuracy drops to 66%, the error rises to 35%. These results enable better understanding of the variations observed in quantum computing experiments and underscore the need for adaptive approaches in quantum error mitigation for ensuring reproducibility on near term devices within the quantum computing community.   

Universal control of a 4 singlet-triplet-qubit quantum processor in a germanium quantum dot ladder

Rich underlying physics and a relatively high-yield fabrication process have enabled Ge/SiGe quantum dot arrays to emerge as a platform for high-fidelity single-spin qubits [1] and singlet-triplet (ST) qubits with all-electrical control of a quantum processor [2]. However, despite the appealing potential, several challenges remain in its implementation, such as readout, control, and scaling of systems with many qubits [3].

In this work, we construct four singlet-triplet qubits in a 2x4 Ge/SiGe quantum dot array. This represents the largest array of ST semiconductor qubits to date, featuring tunable couplings between all sites. We demonstrate coherent control of all qubits i.e. x-axis rotations driven by spin-orbit coupling and z-axis control driven mainly by exchange coupling. The readout is performed via Pauli-spin blockade. We also implement a two-qubit swap gate between the qubit pairs. This is achieved by increasing the exchange coupling between neighbouring ST qubits. Furthermore, we demonstrate the quantum state transfer across the array using a sequence of swaps.

This concludes the characterization of the system and allows us, for example, to use this quantum processor to simulate quantum phase transitions, resonance valence bond states and magnetism of ladder systems.

[1] Hendrickx N. W., et al. Nature, 591 (7851), 580–585, (2021).

[2] Jirovec, D., et al. Nature Materials, 20(8), 1106–1112, (2021).

[3] Vandersypen, L., et al. npj Quantum Inf.3, 34 (2017).   

Coupling of topologically protected singlet-triplet qubits in synthetic spin-one chains realized in an InAsP quantum dot nanowires

Using atomistic theory and exact diagonalization tools we describe a chain of InAsP quantum dots in an InP nanowire with four electrons each. Two electrons in each dot occupy two p-shell orbitals forming a triplet state so that each dot simulates a spin-one object in the low-energy limit and a single chain simulates a synthetic spin-one chain. From atomistic interacting Hamiltonian we derive a Hubbard-Kanamori Hamiltonian. Using exact diagonalization and MPS tools we show that in a range of parameters the system has a low-energy spectrum similar to that of an anti-ferromagnetic spin-one chain, with the low-energy spectrum consisting of a singlet and a triplet (ST) separated by a topological gap from the rest of the spectrum, making the chain a candidate for realizing a robust macroscopic singlet-triplet (ST) qubit. Here we describe different possible couplings of two chains at their ends for realizing two-qubit gates. Previous proposals required tunable ST gap. The ST gap here depends on the chain length and is difficult to dynamically control. We show that this limitation may be overcome using different background magnetic fields for the two chains, avoiding the leakage out of computational basis, and implementing two-qubit gates with high accuracy.   

Embedding of a non-Hermitian Hamiltonian to emulate the von Neumann measurement scheme

The problem of how measurement in quantum mechanics takes place has existed since its formulation. Von Neumann proposed a scheme where he treated measurement as a two-part process -- a unitary evolution in the full system-ancilla space and then a projection onto one of the pointer states of the ancilla (representing the "collapse" of the wavefunction). The Lindblad master equation, which has been extensively used to explain dissipative quantum phenomena in the presence of an environment, can effectively describe the first part of the von Neumann measurement scheme when the jump operators in the master equation are Hermitian. We have proposed a non-Hermitian Hamiltonian formalism to emulate the first part of the von Neumann measurement scheme. We have used the embedding protocol to dilate a non-Hermitian Hamiltonian that governs the dynamics in the system subspace into a higher-dimensional Hermitian Hamiltonian that evolves the full space unitarily. We have obtained the various constraints and the required dimensionality of the ancilla Hilbert space in order to achieve the required embedding that is valid for all time. Using this particular embedding and a specific projection operator, one obtains non-Hermitian dynamics in the system subspace that closely follow the Lindblad master equation. This work lends a new perspective to the measurement problem by employing non-Hermitian Hamiltonian evolution.   

Experimental Quantum Error Correction of Binomial Bosonic Codes

Quantum error correction (QEC) is essential for achieving universal and scalable quantum computation, as quantum systems inevitably interact with the environment. Among various QEC schemes, bosonic encodings in superconducting microwave modes are appealing because of the hardware efficiency of bosonic modes with the intrinsically large Hilbert space for redundant information encoding. Recent years have witnessed significant experimental advancements in implementing bosonic QEC in superconducting circuits. In this talk, I will focus on our recent progress in QEC using binomial bosonic codes [1]. Specifically, I will discuss our successful demonstration of surpassing the breakeven point of QEC [2] and protecting quantum entanglement between two bosonic logical qubits via QEC [3].

[1] M.H. Michael, et al. Phys. Rev. X 6, 031006 (2016).

[2] Ni Z, et al. Nature 616, 56 (2023).

[3] W. Cai, et al. arXiv: 2302.13027 (2023).   

Quantum error mitigation and correction mediated by Yang-Baxter equation and artificial neural network

Artificial error mitigation harmonizes classical and quantum computing, capitalizing on their individual strengths to offset weaknesses. Classical algorithms analyze and model errors in quantum computations, guiding corrective actions on quantum states to enhance reliability without the overhead of traditional error correction codes. This study demonstrates how machine learning and zero-noise extrapolation (ZNE) reduce quantum noise. ZNE faces challenges when generating noisy data for large quantum circuits and qubit systems. Focusing on quantum time dynamics (QTD) simulations, errors from unitary folding (a technique to generate noisy data in ZNE) can lead to undesired results. Also, implementing ZNE at each time step introduces significant overhead. Our solution uses artificial neural networks to master a subset of time steps and rectify the remaining dynamics. Leveraging the Yang-Baxter equation [1, 2, 3] for circuit compression provides control over depth and generates extra noise data without additional numerical errors.   

High-Q quartz mechanical resonator as quantum memory unit

Quantum memory units with lifetime longer than currently available qubits are crucial for quantum computation. Long lifetime and small size make mechanical resonators good candidates for storing quantum information. We have fabricated a suspended 1-D Z-cut quartz phononic crystal resonator presenting a mechanical mode confined in a central defect with frequency around 100MHz and high Q, which we currently measure optically. Our quartz resonator will couple parametrically to a SNAIL (Superconducting Nonlinear Asymmetric Inductive eLement) to mediate the frequency mismatch with GHz-frequency devices. Ultimately the SNAIL will couple to transmon qubits. The quartz resonator and SNAIL are fabricated on separate chips and flip-chip bonded with a few micron gap. Such a small gap will allow us to achieve a large piezoelectric coupling to the quartz resonator via non-contacting electrodes on the SNAIL chip which do not contribute extra mechanical dissipation. This modular integration architecture will improve the flexibility and scalability of the whole hybrid system.   

Modeling the high-pressure solid and liquid phases of tin from deep potentials with ab initio accuracy

Constructing an accurate atomistic model for the high-pressure phases of tin (Sn) is challenging because the properties of Sn are sensitive to pressures. We develop machine-learning-based deep potentials for Sn with pressures ranging from 0 to 50 GPa and temperatures ranging from 0 to 2000 K. In particular, we find the deep potential, which is obtained by training the ab initio data from density functional theory calculations with the state-of-the-art SCAN exchange-correlation functional, is suitable to characterize high-pressure phases of Sn. We systematically validate several structural and elastic properties of the α (diamond structure), β, bct, and bcc structures of Sn, as well as the structural and dynamic properties of liquid Sn. The thermodynamics integration method is further utilized to compute the free energies of the α, β, bct, and liquid phases, from which the deep potential successfully predicts the phase diagram of Sn including the existence of the triple-point that qualitatively agrees with the experiment.   

First-principles study on heat and charge transport of warm dense aluminum with Kohn-Sham and stochastic density functional theory

Traditional Kohn-Sham density functional theory (KSDFT) is one of the most popular quantum-mechanics-based methods in modeling materials since it balances the accuracy and efficiency well. Accurately predicting electron transport coefficients is crucial for understanding warm dense matter. The Kubo-Greenwood formula, within the framework of KSDFT, is commonly employed to evaluate the electrical and thermal conductivities of electrons. However, in some works, the velocity operator of this formula is approximated by the momentum operator without non-local potential corrections. Moreover, traditional KSDFT, based on the diagonalization method, is inefficient for simulating high-temperature systems. Recently, stochastic density functional theory [Phys. Rev. Lett. 111, 106402 (2013)] (SDFT) and its improved theory, mixed stochastic-deterministic density functional theory [Phys. Rev. Lett. 125, 055002 (2020)] (MDFT) are developed based on stochastic orbitals. These methods enable more efficient simulation of high-temperature systems. In this study, we carefully examine this approximation of the velocity operator in warm dense aluminum at temperatures ranging from 0.1 eV to 10 eV and at densities of 2.35 g/cm³ and 2.7 g/cm³, using both pseudopotential with 3 valence electrons and 11 valence electrons. Our findings indicate that considering the non-local potential effect is essential for accurately calculating the heat and charge transport properties of electrons. Furthermore, we observe that the frozen-core approximation is invalid for calculating conductivities before the ionization of corresponding core electrons occurs. Additionally, we develop a new Kubo-Greenwood formula within the framework of MDFT to calculate thermal and electrical conductivities. This work demonstrates that the proposed method is capable of accurately calculating electronic thermal and electrical conductivities at temperatures of hundreds of eV.   

Quantum algorithm for radiative transfer equations

We report a new quantum algorithm for radiative transfer using the lattice Boltzmann method (LBM). This algorithm encompasses all the essential processes of radiative transfer: absorption, scattering, and emission. The algorithm primarily consists of three sections. The first section calculates the effects of absorption (leading to a decrease) and scattering, while the second section computes the effects of absorption (leading to an increase) and emission. These two sections do not depend on the number of grid points, ensuring that the computational complexity does not increase with the number of grid points. The third section computes the propagation processes. Except for the initial encoding and final measurements, this quantum algorithm accelerates radiative transfer calculations on a logarithmic scale compared to classical algorithms. Furthermore, we present simulation results for a test problem using this algorithm. The close agreement between our computational results and analytical solutions demonstrates the validity of this algorithm.   

Effects of Hematocrit and Non-Newtonian Blood Rheology on Pulsatile Wall Shear Stress Distributions in Vascular Anomalies: A Multiple Relaxation Time Lattice Boltzmann Approach

Hematocrit (Hct) is defined as the volume percentage of red blood cells in blood. Hct plays a crucial role in determining the viscosity of blood, and therefore flow patterns within the cardiovascular system. In the present study, a three-dimensional computational technique based on the lattice Boltzmann method (LBM) with a multiple relaxation time (MRT) collision operator is employed to investigate the effect of Hct on blood flow patterns, and wall shear stress (WSS) distributions under pulsatile conditions in vascular anomalies. To accurately represent the non-Newtonian, rheological properties of blood, we developed a constitutive model based on the Carreau-Yasuda model and available published experimental data, where the parameters in the model are a function of Hct. We analyze the numerical and physical aspects of the non-Newtonian MRT-LBM model, coupled with the proposed constitutive equation, to assess the accuracy and numerical stability of the present model and how it is affected by different Hct levels. Our results indicate that changes in Hct levels have a significant impact on flow dynamics in vascular anomalies. Elevated levels of Hct lead to increased oscillatory WSS variations. When the Hct level is increased, the response in the WSS distribution is non-linear. Also, pulsatile blood flow, combined with altered rheological properties due to Hct variations, affects WSS distribution in aneurysms and stenosis.   

Stability of a plane Poiseuille flow in a three-layered anisotropic porous-fluid channel

The modal and non-modal stability analysis of plane Poiseuille flow through a three-layer channel containing a centered anisotropic porous layer parallel to the channel walls is investigated. The channel is confined by solid impermeable walls and governed by the volume-averaged Navier-Stokes equation in the porous layer and the Navier-Stokes equation in fluid layers. At porous-fluid interfaces, continuity of stress and velocity is employed, while no-slip conditions are used at impermeable walls. A modal stability analysis is performed to comprehend the long-time flow transition characteristics. Note that the eigenvalue-based modal stability analysis only describes the asymptotic fate of the perturbation and thus fails to capture the short-term characteristics of the flow. In contrast, the non-modal stability analysis determines the perturbation response at a short time and the transient growth of the perturbations. The present study shows that the system parameters, such as porosity, anisotropic permeability, and porous layer thickness, significantly affect the long- and short-time stability characteristics of the flow that leads to energy growth. The present analysis provides a valuable means to control the flow instability of a multi-layer porous system having anisotropic permeability.   

Fluid-Structure Interactions in Flexible Airfoils: A Combined Approach Using DIC and PIV Techniques

Fluid-structure interaction response of a highly flexible high aspect ratio airfoil was investigated through a series of water tunnel experiments, employing a cutting-edge measurement technique that combines digital image correlation (DIC) and particle image velocimetry (PIV). Throughout the measurement process, we concurrently captured both flow and structure tracers through high-speed imaging, which were subsequently integrated to provide synchronized flow field and dynamic structural response data. Our primary focus was on studying flow-induced vibration response in flexible airfoils under diverse operational conditions. This included investigating angles of attack spanning from 0° to 20° and exploring a broad range of flow velocities. Our findings revealed a dynamic response that exhibited limit cycle oscillation patterns, with different modes of vibration, including bending, torsional, and edgewise modes, depending on the operational conditions, such as changes in angle of attack or flow velocity. Additionally, at higher angles of attack and flow velocities, the airfoil demonstrated multifrequency oscillations, involving various modes contributing to its overall response.   

Soft makes it hard to swim: Role of micro-confinement softness in active swimmer dynamics

Biological microswimmers alter their motility in response to external cues like chemical concentration, boundary proximity, and ambient flow. However, the response of self-propelled microswimmers to the stiffness of a confining wall in their vicinity, and their dynamics remain ambiguous. Here, we experimentally investigate the effects of varying softness of the micro-confinement walls on the swimming dynamics of self-propelled microswimmers, considering active droplets as a model system. We note dramatic differences in the swimming dynamics of these droplet microswimmers in a rigid micro-confinement, and in a soft-walled micro-confinement of identical geometry. While for the rigid case the active droplets exhibit steady unidirectional swimming velocity along a confinement wall, in case of soft confinements the microswimmer exhibits unsteady swimming dynamics with intermittent deceleration, stopping, and subsequent acceleration. These changes in the self-propulsion velocity are accompanied by sharp reorientation of the swimming direction. Interestingly, the microswimmer exhibits autonomous changes in its flow field characteristics over such stopping and acceleration events. We use high-resolution microscopy and micro-PIV analysis to characterize such dynamics. Our study will aid the understanding of bio-physical response of swimming micro-organisms to mechanical cues from the confinement walls, and conceptualization of active cargo-delivery in soft, complex environment.   

Li3VO4 (LVO) Nanosheets as a Long-life (>10,000) and High-rate Anode for its application in Sodium-ion Battery (SIBs)

Lithium-based transition metal oxides, such as LVO, are known for their exceptional capacity and intrinsic safety features, making them promising materials for lithium-ion batteries. Unfortunately, LVO's practicality is limited by its moderate lifespan under high rates [1]. The study shows that solution-processed laminar crystalline LVO (calculated bandgap: 3.95 eV), without surfactants, exhibits unparalleled high-rate performance and ultra-long cycling stability with Na⁺-ions. The X-ray diffraction analysis of LVO powder revealed that it has an orthorhombic phase with lattice constants of a=5.448 Å, b=6.327 Å, and c=4.949 Å, with the Pmn21 space group [2]. In the cyclic voltammogram (CV) test, LTO||Na showed two pairs of oxidation and reduction peaks at [1.22 & 1.43 V] and [0.254 & 0.851 V], respectively, at a lower scan rate of 0.1 mV/s. However, at 6 mV/s, the overall CV profile showed only one prominent pair of the redox couple, 1.59/0.887 V. Furthermore, LVO||Na cell delivers an initial 202 mAh/g capacity at 0.25 A/g with a capacity retention (CR) of 73.26 % after 1600 cycles during GCD cycling. Under the high current density of 5 A/g, the cell initially delivers a capacity of 51.3 mAh/g (CR: 95.5 % ) after 10,000 cycles. There is a 34.38 % decrease in the intercalation potential of the LVO||Na when we move from low to high current density. The periodic rate capability test results indicate a ~2.4% increase in the specific discharge capacity for a 0.1 C rate after repeating the cycle from 0.1 C to 10 C, with 20 cycles each. It is noted in the study that LVO||Na demonstrates an increase in overall capacity primarily due to the intercalation of larger ions compared to lithium, which is mainly due to the increased interlayer spacing between the sheets, which consequently increases the surface area for further intercalation. Furthermore, techniques such as time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to understand better the mechanism behind LVO||Na cell's performance.   

Ferroelectric Phase Transitions in Strain-Free Barium Titanate (BaTiO3) Membranes: An Attempt for Pyroelectric Applications

The ferroelectric transition in Barium Titanate (BaTiO3) serves as a paradigmatic example of a phase transition altered by strain. In bulk materials, this transition is typically considered a first-order transition, primarily due to a substantial latent heat exchange associated with structural changes. Conversely, in epitaxial thin films, the presence of strain results in a continuous transition, accompanied by a comprehensive shift in thermodynamics. This study investigates the nature of this transition on free-standing BaTiO3 thin film membranes obtained through sacrificial, water-soluble layers to release the oxide films from the substrate. We use nano-calorimetric measurements and lattice parameter measurements within ferroelectric membranes, where strain induced by the substrate is absent. We explore the nature of the phase transition and both the similarities and inconsistencies observed in X-ray diffraction (XRD), Raman spectroscopy, and nano-calorimetric measurements. Additionally, preliminary pyroelectric measurements during rapid temperature changes in a ferroelectric membrane-based capacitor system are presented. The findings from both sets of measurements contribute to our understanding of the separate effects of strain and finite size on ferroelectric transitions in thin films, as well as their potential practical applications.   

A New Physical Model and Anti-Fog Technology for Undistorted Visualization through Endoscope Lenses using Sterile, Benign, Hyper-Hydrophilic Gel Coatings to Planarize and Absorb Condensing Water

Fogging on endoscope lenses limits visualization in closed body cavity surgery, due to differences in body temperatures vs Operating Rooms (OR). Surgeons have to remove and reinsert endoscopes, which increases infections, scarring, surgery durations, and costs.

Fogging is due to discontinuous 3D films formed by (hemi-) spherical drops refracting light.

The present anti-fog model and technology, KnoxFog™, uses hyper-hydrophilic [1] coatings. Hyper-hydrophilicity combines super-hydrophilicity [2-4] with "absorbing nanopores".

Super-hydrophilicity condenses water into continuous, flat, clear 2-D film [2] . Absorbing nanopores consist of a uniform hydrophilic network of nanopores drawing molecules into the material, which evacuates condensating water by molecular absorption. Combining super-hydrophilicity with absorbing nanopores maintains condensating water films flat and uniform, free of optical distortion, and inhibits water runoff into large drops. Hyper-hydrophilicity impedes two types of droplet formation – fog and run-off.

Current anti-fog strategies are insufficient. Acidic and alcohol-based anti-fog solutions quickly evaporate. Heating endoscopes requires reheating every 5 to 40 min [2, 4-8].

Hyper-hydrophilic KnoxFog™ is applied as a solid gel via a small, sterile, reusable, applicator, ClearEndoscope™, and dries in seconds.

In vitro comparison with eight endoscopes shows imaging is fog-free for hours with KnoxFog™ vs bare lenses, lens heating and other anti-fogs.   

Characterizing the Brain with Parametric Mapping of Divergence in Phase-Shift Sensitive Magnetic Resonance Imaging

Phase shift sensitive magnetic resonance imaging, such as multi-direction diffusion imaging, where many directional measurements at varying angles are acquired at each pixel/voxel location, is widely relied upon in research and in the clinic. Analytic methods for parametric mapping, such as scalar image reconstruction, are widely used, producing a variety of image types such as fractional anisotropy, the apparent diffusion coefficient, kurtosis imaging, etc. In this study, we present two novel scalar image reconstruction techniques for parametric mapping of multi-direction phase shift sensitive magnetic resonance imaging examinations, producing phase shift divergence images, and associated sink/source images. The techniques developed were evaluated on a large (n=642) clinical neurological magnetic resonance imaging (MRI) dataset. Results demonstrate potential for novel characterization of water movement properties through these two new image types, which can complement existing techniques. These new images may be sensitive to water flow and microstructural architecture of the brain. While potential utility is demonstrated on a large clinical MRI dataset with statistical analyses, extensive validation is required to confirm utility and validity as part of future work.   

An Empirical Extension of the Ridge Regression Theorem to Nonlinear Least Squares Biexponential Analysis, with Application to Myelin Mapping in the Human Brain

Nonlinear least squares (NLLS) is a highly effective means of parameter estimation but can be extremely sensitive to noise in situations where the estimation problem is poorly-posed. To decrease the mean square error (MSE) of parameter estimation in such circumstances, we apply Tikhonov regularization, even though this is highly unconventional for low-dimensional NLLS problems with parameter estimates of qualitatively different types of parameters. In this sense, we are extending the use of the ridge regression theorem (RRT) for linear least squares, which states that a regularization parameter, λ, exists that reduces MSE in parameter estimation as compared with nonregularized linear least squares. We estimated parameter values with conventional NLLS and compared these with values obtained from regularized NLLS, with λ defined by generalized cross validation. We only regularized signals identified as biexponential by the Bayesian information criterion. Under conditions of modest SNR and relatively closely spaced exponential time constants, regularization substantially reduces variance and MSE across noise realizations. We applied this method to the challenging case of myelin mapping in the brain and found a 20% improvement in MSE. Finally, we note that our method is generalizable to many other NLLS analyses.   

Localized Regularization for Solving the Fredholm Equation of the First Kind with Application to Brain Myelin Mapping

Magnetic resonance relaxometry (MRR) studies of the brain are used to map the distribution of myelin, a complex lipid that forms an insulating sheath around axons and potentiates normative electrical transmission. Measurement of the distribution function (DF) of transverse relaxation times, or T2, can be used to quantify the myelin water fraction (MWF). This involves solving the Fredholm integral equation of the first kind with a Laplace kernel, a notoriously ill-posed problem, with solutions very sensitive to noise. Tikhonov regularization is used to stabilize the determination of the T2 DF, with an optimal regularization parameter, λ, selected according to one of several methods. However, this choice generally represents a compromise, with certain regions of the DF being over-regularized and others being under-regularized. To address this major shortcoming, we present a novel parameter selection method, named localized regularization (LocRegu), that iteratively tunes a vector of λ values across the entire DF, so that each part of the DF may be optimally regularized. We demonstrate the superior performance of LocRegu in simulated data, classical inverse problems, and experimental MRR data for brain MWF determination.   

Developing Recurrent Neural Networks to Predict Gait Speed with Longitudinal Clinical Information

Gait speed is recognized as a significant indicator of biological aging. Through the development and use of tools that identify accelerated aging trajectories and their associated biomarkers, clinical decisions can be better-informed to include earlier and more effective interventions. Recurrent deep learning models allow for the investigation of longitudinal, nonlinear relationships between clinical variables and outcomes. The purpose of this work is to develop a recurrent neural network (RNN) to predict aging-related incident slow gait and its determinants across various timeframes from a basic set of health measures. By comparing the longitudinal analysis of an RNN with the analysis of a non-longitudinal neural network (NN), we intend to determine the relevance of longitudinal information in predictions of aging-related decline. We are utilizing the 3,821 gait speed measurements from 1,363 unique subjects in the Baltimore Longitudinal Study of Aging (BLSA) and a clinically relevant gait speed cut-point (1.0 m/s) to investigate the prediction of both current and future (2-year and 6-year timeframes) slow gait. Currently, the RNN has not demonstrated an improvement in performance over the NN for each of the current and future predictions. Going forward, we are developing new RNN architectures and exploring additional variables to identify the determinants of gait speed and determine the significance of longitudinal information in the BLSA.   

Growth and Visualization of an MgZn2-Zn Spiral Eutectic Using Molecular Dynamics Simulation

The intrinsic chirality of spiral eutectics offers the potential to speed up the manufacture of large-area photonic materials. Molecular Dynamics simulations were performed on an Mg-Zn system to analyze the formation and growth of an MgZn₂-Zn spiral eutectic. Simulations were run on a 33 at.% Mg and 66 at. % Zn system to reproduce the growth mechanism of the MgZn₂ crystal. Simulations were also performed on the typical experimental composition of 92 at. % Zn 8 at. % Mg to model the crystallization of the MgZn₂ and Zn phases together. Using this data, we provide a critical assessment of the proposed mechanisms for the growth of these spiral formations.   

Numerical simulations of neuromorphic behavior in Mott memristive devices

The information age we live in is supported on a physical under-layer of electronic hardware, which originates in condensed matter physics research. The mighty progress made in silicon based technology seemed endless.  However, with the smallest feature size of transistors reaching down to mere 5 nm and the large power requirements,

this technology is reaching an unavoidable physical limit. This calls for exploration of new alternatives.

Neuromorphic inspired systems are making fast progress. But they is based either on hardware made with conventional CMOS electronics, or in software, such Deep Neural Networks algorithms that run in neuromorphic chips, which are optimized conventional computers.

Resistive switching phenomena, also known as memristive behavior, opens the way to explore a technological disruptive solution [1]. Namely, to implement simple devices with the required neuromorphic functionalities of neurons and synapses, which would allow to directly build in hardware neural networks systems for artificial intelligence. A an exciting development was to realize that Mott quantum materials can be used to implement artificial spiking neurons. However, the understanding of Mott insulators driven out of equilibrium remains a significant challenge [2]. In this talk we shall describe recent efforts towards making artificial neurons using Mott strongly correlated systems and modeling their facsinating emergent behavior [3].

[1] Challenges in materials and devices for resistive-switching-based neuromorphic computing;

Javier del Valle, Juan Gabriel Ramírez, Marcelo J. Rozenberg, and Ivan K. Schuller; Journal of Applied Physics 124, 211101 (2018)

[2] Subthreshold firing in Mott nanodevices

J. Del Valle, et al.; Nature [01 May 2019, 569(7756):388-392]

[3] Exponential Escape Rate of Filamentary Incubation in Mott Spiking Neurons

Rodolfo Rocco, Javier del Valle, Henry Navarro, Pavel Salev, Ivan K. Schuller, and Marcelo Rozenberg

Phys. Rev. Applied 17, 024028 (2022)

Editors’ Suggestion   

Strangeness Enhancement in Event Generators

Cosmic ray air showers and their associated cascades of hadronic daughter particles are often studied using Monte Carlo simulations of hadron-nucleus collisions. However, these simulations often fail to account for high muon fluxes observed in data collected from cosmic ray observatories (such as the Pierre Auger Observatory), a problem known as the Muon Puzzle. In this work, I study how such event generators (namely Pythia 8.308, EPOS-LHC, and Sibyll-2.3) account for strangeness enhancement (increased production of strange hadrons), a possible physical explanation for the Muon Puzzle at TeV energies. By simulating up to 100,000 proton-proton collisions at both 7 TeV and 13 TeV energies in all three event generators, I computed charged particle multiplicity classes based on definitions used in experimental analyses conducted by the ALICE collaboration (2017). I then computed yields ratios of strange particle counts for all three event generators at both energy levels for each multiplicity class to investigate whether strangeness enhancement was simulated in these event generators, which would show up as an increase in strangeness as a function of the multiplicity class. As expected, strangeness enhancement was observed in EPOS-LHC and not observed in Pythia 8.308 and Sibyll-2.3, where this effect is not physically modeled. These results and the analysis code that I wrote will be used in future LHCb studies and similar collider experiments to compare simulated strangeness with experimental strangeness production in data and will help to discern which event generators are useful representations of real-life hadron collision physics.