Papers

Electron-photon hybrid systems serve as ideal light-matter interfaces with broad applications in quantum technologies. These systems are typically operated dynamically under nonequilibrium conditions, giving rise to coupled electronic and photonic currents. Understanding the joint fluctuation behavior of these currents is essential for assessing the performance of light-matter interfaces that rely on electron-photon correlations. Here, we investigate the full counting statistics of coupled electronic and photonic currents in an experimentally feasible hybrid system composed of a double quantum dot coupled to an optical cavity. We employ the framework of quantum Lindblad master equation which is augmented with both electronic and photonic counting fields to derive their joint cumulant generating function–a treatment that differs significantly from existing studies, which typically focus on either electron or photon statistics separately. We reveal that the ratio between photonic and electronic currents, as well as their variances, can deviate from an expected quadratic scaling law in the large electron-photon coupling regime. Furthermore, we demonstrate that conventional modelings of photonic dissipation channels in quantum master equations must be modified to ensure that the joint cumulant generating function satisfies the fluctuation symmetry enforced by the fluctuation theorem. Our results advance the understanding of joint fluctuation behaviors in electron-photon hybrid systems and may inform the design of efficient quantum light-matter interfaces.

The existence of self-correcting quantum memories in three dimensions is a long-standing open question at the interface between quantum computing and many-body physics. We take the perspective that large contributions to the entropy arising from fine-tuned spatial symmetries, including the assumption of an underlying regular lattice, are responsible for fundamental challenges to realizing self-correction. Accordingly, we introduce a class of disordered quantum codes, which we call "cored product codes". These codes are derived from classical factors via the hypergraph product but undergo a coring procedure which allows them to be embedded in a lower number of spatial dimensions while preserving code properties. As a specific example, we focus on a fractal code based on the aperiodic pinwheel tiling as the classical factor and perform finite temperature numerical simulations on the resulting three-dimensional quantum memory. We provide evidence that, below a critical temperature, the memory lifetime increases with system size for codes up to 60000 qubits.

We explore the molecular beam epitaxy synthesis of superconducting aluminum thin films grown on c-plane sapphire substrates at cryogenic temperatures of 6 K and compare their behavior with films synthesized at room temperature. We demonstrate that cryogenic growth increases structural disorder, producing crystalline grains that modify the optical, electrical, and superconducting properties of aluminum. We observe that cryogenic deposition changes the color of aluminum from fully reflective to yellow and correlate the pseudo-dielectric function and reflectance with structural changes in the film. We find that smaller grain sizes enhance the superconductivity of aluminum, increasing its critical temperature and critical field. We then estimate the superconducting gap and coherence length of Cooper pairs in aluminum in the presence of disorder. Finally, we fabricate superconducting microwave resonators on these films and find that, independently of the growth temperature, the system is dominated by two-level system loss with similar quality factors in the high and low power regimes. We further measure a higher kinetic inductance in the cryogenically grown films.

The statistics of correlations are central quantities characterizing the collective dynamics of recurrent neural networks. We derive exact expressions for the statistics of correlations of nonlinear recurrent networks in the limit of a large number N of neurons, including systematic 1/N corrections, in the regime of Gaussian quenched disorder. Our approach uses a path-integral representation of the network stochastic dynamics, which reduces the description to a few collective variables and enables efficient computation. This generalizes previous results on linear networks to include a wide family of nonlinear activation functions, which enter as interaction terms in the path integral. These interactions can resolve the instability of the linear theory and yield a strictly positive participation dimension. We present explicit results for power-law activations, revealing scaling behavior controlled by the network coupling. In addition, we introduce a class of activation functions based on Pade approximants and provide analytic predictions for their correlation statistics. Numerical simulations confirm our theoretical results with excellent agreement. We also compare with previous works that have studied the complementary case with annealed disorder, and based on this we propose a new self-consistent equation for the more general case of colored noise.

The linear combination of atomic orbitals (LCAO) method uses a small basis set in exchange for expensive matrix element calculations. The most efficient approximation for the matrix element calculations is the two-center approximation (2CA) in tight binding (TB). In the 2CA, a variety of matrix elements are neglected with only "two-center integrals" (2CI) remaining. The 2CI are calculated efficiently by rotating to symmetrical coordinates where the integral is parameterized. This makes TB fast in exchange for diminished transferability. An ideal electronic structure method has both the efficiency of TB and the transferability of ab-initio methods. In this work, I expand the full crystal potential into multipoles where the resulting matrix elements are transformed into the form of 2CI between high angular momentum functions. The usual Slater-Koster formulae for TB are limited to $l\leq3$; to enable efficient evaluation of the full crystal potential 2CI, I derive a Wigner matrix based convolution algorithm (WMCA) that works for arbitrary angular momentum. Given a suitable method for generating a local ab-initio Kohn-Sham potential, the algorithm for calculating matrix elements is applicable to fully ab-initio LCAO methods (this is the subject of forthcoming work). In this paper, I apply the WMCA to silicon using a model crystal potential.

The mechanical performance of fiber-reinforced concrete is critically dependent on the orientation and concentration of its reinforcing fibers. This paper presents a non-destructive method for diagnosing fiber parameters in large planar specimens. The proposed technique utilizes an electromagnetic measurement system using electrical coils with commercial LCR meters, but also a novel custom-made meter suitable for factory environments, offering a practical solution for quality control in prefabrication of high-performance structural elements. This study details the theoretical background, experimental setup and methodology, and provides an evaluation of the diagnostic system's effectiveness and accuracy. The results demonstrate a strong correlation between the electromagnetic measurements and the actual fiber parameters, confirming the method's reliability.

Self-propulsion of particles is typically explained by phoretic mechanisms driven by externally imposed chemical, electric, or thermal gradients. In contrast, chemical reactions can enhance particle diffusion even in the absence of such external gradients. We refer to this increase as active diffusivity, often attributed to self-diffusiophoresis or self-electrophoresis, although these mechanisms alone do not fully account for experimental observations. Here, we investigate active diffusivity in catalytic Janus particles immersed in reactive media without imposed gradients. We show that interfacial reactions generate excess surface energy and sustained interfacial stresses that supplement thermal energy, enabling diffusion beyond the classical thermal limit. We consistently quantify this contribution using both dissipative and non-dissipative approaches, assuming that the aqueous bath remains near equilibrium. Our framework reproduces experimentally observed trends in diffusivity versus activity, including the non-monotonic behaviors reported in some systems, and agrees with data for nanometric Janus particles catalyzing charged substrates as well as vesicles with membrane-embedded enzymes driven by ATP hydrolysis. These results demonstrate that chemical reactions can induce and sustain surface-tension gradients and surface excess energy, providing design principles for tuning mobility in synthetic active matter.

The electronic and optical properties of CdGa2Te4 and ZnGa2Te4 were studied using first-principles DFT calculations. Band gaps were calculated using the GGA-PBESol functional. Both materials show promise for photovoltaic applications because of their large, near-unity absorption efficiencies (10^4 cm^-1) in the visible region. They exhibit low exciton binding energies (18.85-26.81 meV), large Bohr radii (23-34.3 Angstrom), and moderate exciton temperatures (218-311 K), which are favorable for photovoltaic applications. Their performance as solar cells was simulated using the SCAPS-1D tool for thin-film devices with Pt/CdS/CdGa2Te4/Cu2O/Ti and Pt/CdS/ZnGa2Te4/Cu2O/Ti structures. We investigated the effects of layer thickness, donor and acceptor concentrations (shallow donors/acceptors), and defect density on device performance. The ideal absorber thickness for XGa2Te4 (X = Cd, Zn) was found to be 1000-1800 nm, and the CdS buffer layer around 100 nm. To obtain an efficiency above 20%, the defect density in the CdGa2Te4 and ZnGa2Te4 absorber layers should be kept below 1.772 x 10^13 cm^-3. The best simulations show efficiencies of 18.46% and 17.35% for CdGa2Te4- and ZnGa2Te4-based solar cells, respectively.

We compare six elastic models for polymer networks in the context of phase separation within a gel, including a new model that combines the finite extensible Arruda-Boyce model and the slip tube model for entangled chains. We study incompressible uniaxial stretch and compression, and three volume-changing constrained-dimension deformations, in which the material can only deform in the designated dimensions(s) while the constrained direction(s) remain(s) the same. Each model responds differently to large deformations, and our proposed model successfully describes both strain softening and strain hardening, which are both present in well-entangled elastomers. When considering phase separation, we show that the commonly-used neo-Hookean model fails to admit a common tangent construction for phase coexistence for 3D deformations. This can be resolved by using a model with finite extension, such as the Arruda-Boyce model. In constrained-dimension deformations, where the gel's volume is allowed to change, for elastic models in which phase coexistence is possible, the critical temperatures increases and the critical concentration decreases with increasing deformation dimensions. This strong dependence of the phase diagram on spatial dimension and geometry distinguishes phase separation elastic media from conventional phase separation.

Time-resolved and ultrafast electron energy-loss spectroscopy (EELS) is an emerging technique for measuring photoexcited carriers, lattice dynamics, and near-fields across femtosecond to microsecond timescales. When performed in either a specialized scanning transmission electron microscope or ultrafast electron microscope (UEM), time-resolved and ultrafast EELS can directly image charge carriers, lattice vibrations, and heat dissipation following photoexcitation or applied bias. Yet recent advances in theoretical calculations and electron optics are often required to realize the full potential of ultrafast EEL spectrum imaging. In this review, we present a comprehensive overview of the recent progress in the theory and instrumentation of time-resolved and ultrafast EELS. We begin with an introduction to the technique, followed by a physical description of the loss function. We outline approaches for calculating and interpreting ground-state and transient EEL spectra spanning low-loss plasmons to core-level excitations analogous to X-ray absorption. We then survey the current state of time-resolved and ultrafast EELS techniques beyond photon-induced near-field electron microscopy, highlighting abilities to image carrier and thermal dynamics. Finally, we examine future directions enabled by emerging technologies, including electron beam monochromation, in situ and operando cells, laser-free UEM, and high-speed direct electron detectors. These advances position time-resolved and ultrafast EELS as a critical tool for uncovering nanoscale dynamic processes in quantum materials and solar energy conversion devices.

Recent interest in quantum materials has focused on systems exhibiting both superconductivity and non-trivial band topology as material candidates to realize topological or unconventional superconducting states. So far, superconductivity in most topological materials has been identified as type II. In this work, we present magnetotransport studies on the quasi-two-dimensional type I superconductor $\mathrm{YbSb_2}$. Combined ab initio DFT calculations and quantum oscillation measurements confirm that $\mathrm{YbSb_2}$ is a Dirac nodal line semimetal in the normal state. The complex Fermi surface morphology is evidenced by the non-monotonic angular dependence of both the quantum oscillation amplitude and the magnetoresistance. Our results establish $\mathrm{YbSb_2}$ as a candidate material platform for exploring the interplay between band topology and superconductivity.

Recent ab initio calculations predict 1D dispersive electronic bands confined to the atomic scale cores of dislocations in the wide bandgap (3.84 eV) semiconductor ZnS. We test these predictions by correlating sub-bandgap optical transitions with the density of dislocations formed during strain relaxation in epitaxial ZnS grown on GaP. The densities for four predicted partial dislocations are quantified using scanning electron microscopy-based electron channeling contrast imaging. Room-temperature ellipsometry reveals absorption peaks that scale with dislocation density and align with theoretical predictions. Low-temperature photoluminescence spectra show deep emission peaks matching dislocation 1D band-to-band transitions. Photoluminescence excitation spectroscopy reveals six distinct emission lines with contrasting excitation dependence. Four peaks (2.78, 2.41, 2.20, 1.88 eV), assigned to dislocations, exhibit only modest suppression ($\leq$5$\times$) when excited below the ZnS bandgap, while two other peaks (3.11, 1.53~eV) are strongly quenched ($>$10$\times$). These findings support the existence of efficient, 1D band-to-band radiative transitions within quantum wire-like dislocation core states in ZnS, distinct from typical non-radiative deep-level defects in wide-gap semiconductors.

Decrease in the size of integrated circuits (IC) and metal interconnects raise resistivity due the amplification of electron scattering effects, which decreases the efficiency of chiplets. While previous studies have investigated the electron scattering due to a roughened surface, the effect of thermal induced atomic motion on the roughened surface remains unclear. To address this gap, we investigated electron transport in pristine and roughened Cu thin films by performing ab initio molecular dynamics (AIMD) trajectories over 20~ps at temperatures of 218~K, 300~K, and 540~K on Cu thin film models, and then calculating the electron transport properties of the resulting snapshots at 100-fs intervals for the last 10~ps using the non-equilibrium Green's function formalism in combination with density functional theory (NEGF-DFT). As expected, higher temperatures induce larger atomic displacement from their equilibrium positions and increase atomic layer separation. We also find that increase in temperature results in increased resistance (lower conductance) for the pristine film, but less so for the roughened thin film where the surface roughness itself is the main source of resistance. This study provides insights into how pristine and roughened Cu thin films behave under thermal conditions, helping researchers design better treatments to mitigate thermal effects in ICs and their metal interconnects.

Measurement of a charge qubit via point contacts with complex internal structures is considered. In this context, a fully formalized derivation of the many-body wave function method is presented, together with the corresponding master equations for point contacts possessing an arbitrary number of internal states. The focus is placed on the current noise power spectrum and its dependence on the qubit dynamics and the point contact parameters.

We present an exact solution for entanglement entropy for the real-time dynamics following a quench from a thermal pure quantum (TPQ) state in a free-fermion system. In contrast to the usual linear growth and saturation behavior, the entanglement entropy exhibits a characteristic double-plateau structure. We establish this behavior through three complementary approaches: an exact conformal field theory calculation on the Klein bottle, finite-size Gaussian-state simulations, and a quasiparticle picture that becomes quantitatively accurate in the scaling regime.

Dopants can tune the performance of MoS2 in various applications, but use of molecular dynamics simulations for doped MoS2 materials discovery is limited by the lack of multi-dopant interatomic potentials. Universal machine learning interatomic potentials (MLIPs) could be a solution, but the accuracy of these potentials must first be evaluated. Here, we evaluate the accuracy of a recently developed MLIP, META's Universal Model for Atoms (UMA), for 25 different MoS2 dopants spanning metals, non-metals, and transition metals in Mo substitution, S substitution, and intercalated positions by benchmarking the MLIP-predicted formation energy and the dopant-induced structural change against density functional theory calculations. The computational framework for MLIP validation and simulations are described in detail and the source code is made open source. The MLIP is then demonstrated by performing heating-cooling simulations of MoS2 supercells with all 25 dopants. These simulations capture complex phenomena including dopant clustering, MoS2 layer fracturing, interlayer diffusion, and chemical compound formation at orders-of-magnitude reduced computational cost compared to density functional theory. This work provides a computational workflow for application-oriented design of doped-MoS2, enabling high-throughput screening of dopant candidates and optimization of compositions for targeted tribological, electronic, and optoelectronic performance. Github Repo Link: https://github.com/AbrarFaiyad/Machine-Learning-Interatomic-Potential-UMA-Enables-MD-Simulations-of-MoS2-Doped

Solar thermochemical water splitting enables hydrogen production by cycling metal oxides between reduced and oxidized states, typically through an oxygen vacancy mechanism. However, recent experimental work suggests that cation vacancies have a greater influence on the redox behavior of iron aluminate spinels used in water splitting. This remains debated, as calculations predict that such cation vacancies are thermodynamically unfavorable. In the current work, we show that Fe vacancies in (Fe$\zeta$Al1-$\zeta$)3O4 become accessible only when facilitated by inversion between Fe and Al. This antisite disorder lowers the formation energy of octahedral Fe vacancies in Al-rich spinels ($\zeta$ = 1/3) from over 3 eV to just 0.62 eV when one third of the cation sites are inverted, allowing high Fe vacancy concentrations under oxidizing conditions. This mechanism supports high H2 yields up to 361 $\mu$mol/g, consistent with experimental observations. Our findings support the notion that solar thermochemical water splitting can occur through a cation vacancy mechanism. They also clarify how site inversion, vacancy energetics, and defect interactions each contribute to redox performance, offering general design principles for identifying and optimizing materials that operate through cation vacancy cycling.

La$_{1-x}$Sr$_{x}$CoO3-$\delta$ is a promising off-stoichiometric metal oxide that undergoes a topotactic perovskite ($\delta$ = 0) to brownmillerite ($\delta$ = 0.5) transition under electrochemical and thermochemical stimuli, with concomitant variations in its electrical, magnetic, thermal, and optical properties. Recent studies on thin-film cycling in electrochemical devices show incomplete reversibility of this transition, with significant acid-etching serving as a degradation mechanism. While earlier investigations examined the protonation of brownmillerite SrCoO2.5, the thermodynamics of protonation across the perovskite-to-brownmillerite transition remain poorly understood. In this work, we combine density functional theory calculations with predictions from universal machine-learning interatomic potentials to elucidate the energetics and implications of protonation across the transition for La0.5Sr0.5CoO3-$\delta$. These calculations reveal negative hydrogen insertion energies and strong competition with oxygen vacancy formation across the transition for a wide range of conditions. The extent of protonation is primarily limited by the availability of Co 3d states to accommodate reduction by inserted hydrogen. Although hydrogen insertion is often thermodynamically favorable within a defect picture, a convex hull analysis of the resulting HyLa0.5Sr0.5CoO3-$\delta$ phases reveals them to be unstable against decomposition into hydroxides among other products. This instability increases with hydrogen content and provides a thermodynamic basis for the acid-etching observed during electrochemical cycling. This work advances the fundamental understanding of protonation in La0.5Sr0.5CoO3-$\delta$ and contextualizes experimental observations of related materials in the presence of moisture or H2.

A Berry phase of odd multiples of $\pi$ inferred from quantum oscillations (QOs) has often been treated as evidence for nontrivial reciprocal space topology. However, disentangling the Berry phase values from the Zeeman effect and the orbital magnetic moment is often challenging. In centrosymmetric compounds, the case is simpler as the orbital magnetic moment contribution is negligible. Although the Zeeman effect can be significant, it is usually overlooked in most studies of QOs in centrosymmetric compounds. Here, we present a detailed study on the non-magnetic centrosymmetric $\mathrm{SrGa_2}$ and $\mathrm{BaGa_2}$, which are predicted to be Dirac nodal line semimetals (DNLSs) based on density functional theory (DFT) calculations. Evidence of the nontrivial topology is found in magnetotransport measurements. The Fermi surface topology and band structure are carefully studied through a combination of angle-dependent QOs, angle-resolved photoemission spectroscopy (ARPES), and DFT calculations, where the nodal line is observed in the vicinity of the Fermi level. Strong de Haas-van Alphen fundamental oscillations associated with higher harmonics are observed in both compounds, which are well-fitted by the Lifshitz-Kosevich (LK) formula. However, even with the inclusion of higher harmonics in the fitting, we found that the Berry phases cannot be unambiguously determined when the Zeeman effect is included. We revisit the LK formula and analyze the phenomena and outcomes that were associated with the Zeeman effect in previous studies. Our experimental results confirm that $\mathrm{SrGa_2}$ and $\mathrm{BaGa_2}$ are Dirac nodal line semimetals. Additionally, we highlight the often overlooked role of spin-damping terms in Berry phase analysis.

Spin qubits in semiconductor quantum dots offer a gate-tunable platform for quantum information processing. While two-qubit interactions are typically realized through exchange coupling between neighboring spins, coupling spin qubits to photons via hybrid spin-cQED devices enables long-range interactions and integration with other cQED platforms. Here, we investigate hole spin-photon coupling in compact single quantum dot setups. By incorporating ubiquitous strain inhomogeneities to our theory, we identify three main spin-photon coupling channels: a vector-potential-spin-orbit geometric mechanism–dominant for vertical magnetic fields–, an inhomogeneous Rashba term generalizing previous spin-orbit field models, and strain-induced $g$-tensor terms–most relevant for in-plane fields. Comparing Si, unstrained (relaxed) Ge, and biaxially strained Ge wells, we find that Si and unstrained Ge provide optimal coupling strengths (tens of MHz) thanks to their reduced heavy-hole, light-hole splitting. We demonstrate efficient switching of the spin-photon coupling while preserving sweet spot operation. Finally, we evaluate quantum state transfer and two-qubit gate protocols, achieving $>99\%$ fidelity for state transfer and $>90\%$ for two-qubit gates with realistic coherence times, establishing single-dot hole spins as a viable platform for compact spin-cQED architectures and highlighting unstrained Ge as a promising candidate for spin-photon interactions.

For the signed graph associated to a deep neural network, one can compute the frustration level, i.e., test how close or distant the graph is to structural balance. For all the pretrained deep convolutional neural networks we consider, we find that the frustration is always less than expected from null models. From a statistical physics point of view, and in particular in reference to an Ising spin glass model, the reduced frustration indicates that the amount of disorder encoded in the network is less than in the null models. From a functional point of view, low frustration (i.e., proximity to structural balance) means that the function representing the network behaves near-monotonically, i.e., more similarly to a monotone function than in the null models. Evidence of near-monotonic behavior along the partial order determined by frustration is observed for all networks we consider. This confirms that the class of deep convolutional neural networks tends to have a more ordered behavior than expected from null models, and suggests a novel form of implicit regularization.

The authors present SHarmonic, a new implementation of the spherical harmonics targeted for electronic-structure calculations. Their approach is to use explicit formulas for the harmonics written in terms of normalized Cartesian coordinates. This approach results in a code that is as precise as other implementations while being at least one order of magnitude more computationally efficient. The library can run on graphics processing units (GPUs) as well, achieving an additional order of magnitude in execution speed. This new implementation is simple to use and is provided under an open source license, it can be readily used by other codes to avoid the error-prone and cumbersome implementation of the spherical harmonics.

van der Waals stacking of two-dimensional (2D) materials offers a powerful platform for engineering material interfaces with tailored electronic and optical properties. While most van der Waals multilayers have featured inorganic monolayers, incorporating molecular monolayers introduces new degrees of tunability and functionality. Here, we investigate hybrid bilayers composed of atomically thin perylene-based molecular crystals interfaced with monolayer transition metal dichalcogenides (TMDs), specifically MoS2 and WS2. Using ab initio many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach, we predict emergent properties beyond those of the isolated constituent systems. Notably, we find substantial renormalization of monolayer molecular crystal band gap due to TMD-induced polarization. Furthermore, by varying the TMD monolayer, we demonstrate tuning of the energy level alignment of the bilayer and subsequent control over a diversity of lowest-energy excitons, which include strongly bound hybrid excitons and long-lived charge-transfer excitons. These findings establish organic-inorganic van der Waals heterostructures as a promising class of materials for tunable optoelectronic devices and quantum excitonic phenomena, expanding the design space for low-dimensional systems.

Strong interactions in Landau flat bands are known to stabilize correlated states that do not form in other types of flat bands. We report hallmarks of topological protection at the Landau level filling factor v=7/11 in a two-dimensional electron system. The $\nu=7/11$ filling factor is the particle-hole conjugate of $\nu=4/11$, a filling factor intensely studied for the possibility of realizing unconventional electronic correlations. Our data establishes a new instance for an unusual fractional quantum Hall state and opens up possibilities for the study of unconventional correlations in an enlarged parameter space. We report and discuss transport signatures developing at other filling factors of interest $\nu= 11/17$, $5/8$, and $8/13$, which however in our sample do not exhibit topological protection.

Higher-order interactions that nonlinearly couple more than two nodes are important in many networked systems, and their effects on collective dynamics are increasingly being studied. Here we provide an overview of this rapidly growing field, and of the techniques that can be used to describe and analyze them. We focus in particular on new phenomena and challenges that emerge when nonpairwise interactions are considered. We conclude by discussing open questions and promising future directions on the collective dynamics of higher-order networks.

Nano-patterned semiconductor interfaces offer a versatile platform for creating quantum metamaterials and exploring novel electronic phenomena. In this study, we illustrate this concept using artificial graphene–a metamaterial featuring distinctive properties including Dirac and saddle points. We demonstrate that introducing additional nano-patterning can open a Dirac band gap, giving rise to what we term artificial hexagonal boron nitride (AhBN). The calculated valley Chern number of AhBN indicates the presence of topological valley Hall states confined to Dirac-gap domain walls. A key question is whether these one-dimensional edge states are topologically protected against disorder, given their vulnerability to Anderson localization. To this end, we perform band structure and electronic transport simulations under experimentally relevant disorder, including charge puddles and geometric imperfections. Our results reveal the resilience of the domain wall states against typical experimental disorder, particularly while the AhBN band gap remains open. The localization length along the domain wall can reach several microns–several times longer than the bulk electron mean free path–even though the number of bulk transport channels is greater. To enhance the effectiveness of the low-dissipation domain wall channel, we propose ribbon geometries with a large length-to-width ratio. These findings underscore both the potential and challenges of AhBN for low-energy, power-efficient microelectronic applications.

Bound electron-hole pairs in semiconductors known as excitons are the subject of intense research due to their potential for optoelectronic devices and applications, especially in the realm of two-dimensional materials. While the properties of free excitons in these systems are well understood, a general description of the interactions between these quasiparticles is complicated due to their composite nature, which leads to important exchange processes that can take place between the identical fermions of different excitons. In this work, we employ a variational approach to study interactions between Wannier excitons and obtain an effective interaction potential between two ground-state excitons in a system of spin-degenerate electrons and holes. This potential is in general nonlocal in position space and depends on the combined spin configurations of the electrons and holes. When particularized to the case of hydrogen-like excitons with a heavy hole, this potential becomes local and exactly reproduces the Heitler-London result for two interacting hydrogen atoms. Thus, our result can be interpreted as a generalization of the Heitler-London potential to the case of arbitrary masses. We also show how including corrections due to excited states into the theory results in a van der Waals potential at large distances, which is expected due to the induced dipole-dipole nature of the interactions. Our approach is also applicable to more complicated systems with nonhydrogenic exciton series, such as layered semiconductors with Rytova-Keldysh interactions. Additionally, we use a path-integral formalism to develop a many-body theory for a dilute gas of excitons, resulting in an excitonic action that formally includes many-body interactions between excitons. While in this approach the field representing the excitons is exactly bosonic, we clarify how the internal exchange processes arise in...

The recently-discovered family of Kagome metals has attracted significant interest due to reports of charge-bond order, orbital magnetism, and superconductivity. Some of these phases may exhibit time-reversal symmetry breaking. More recently, experiments have reported the emergence of nematic order that lowers the rotational symmetry of the system from sixfold to twofold. Here we investigate the mechanism behind a nematic loop-current bond order (NLCBO) that breaks both rotational and time-reversal symmetries. Examining an effective patch model that captures one $p$-type and one $m$-type van Hove singularity at each $M$ point, we find that frustration of the complex order-parameter phases leads to NLCBO. We further present conditions for overcoming other competing phases, including isotropic charge-bond and loop-current orders. Applying our findings to a previously studied model for $\mathrm{A}\mathrm{V}_3\mathrm{Sb}_5$ $(\mathrm{A}=\mathrm{K,Rb,Cs})$, we find that NLCBO emerges within a small region of phase space within mean-field theory. Our theory provides a microscopic description that goes beyond symmetry-allowed free-energy analyses and is broadly applicable to other Kagome metals featuring van Hove singularities near the Fermi level.

We have used the auxiliary-field quantum Monte Carlo (AFQMC) many-body approach on the lattice to study the equation of state for a fermionic impurity interacting with a background sea of spin-polarized fermions. The impurity, or polaron, is an interesting system in both cold atomic and nuclear physics. Our approach is general, and we are able to straightforwardly study the polaron across these regimes. We first study the Fermi polaron at unitarity and for a wide range of scattering lengths, comparing against previous theoretical and experimental studies. We then explore the neutron polaron which has been shown to be an important constraint for nuclear physics. We have also employed the recently developed parametric matrix model to emulate AFQMC solutions to the two-body problem on the lattice, to accelerate the tuning of our lattice Hamiltonian parameters directly to two-body energies in a periodic box, following Luscher's formula. Our lattice quantum Monte Carlo results for the polaron in both a cold atomic and nuclear physics context can serve as stringent benchmarks for future theoretical and experimental research.

The presence of a boundary enriches the nature of quantum phase transitions. However, the boundary critical phenomena in topological superconductors remain underexplored so far. Here, we investigate the boundary criticality in a two-dimensional correlated time-reversal-invariant topological superconductor tuned through a quantum phase transition into a trivial time-reversal-breaking superconductor. Using sign-problem-free determinant quantum Monte Carlo simulations, we chart the quantum phase diagram and reveal the boundary criticalities encompassing ordinary, special, and extraordinary transitions. Additionally, using renormalization group analysis, we compute the boundary critical exponent up to two loops. Remarkably, the simulations and two-loop renormalization group calculations consistently demonstrate that the presence of the boundary Majorana fermion at the special transition gives rise to a new type of boundary Gross-Neveu-Yukawa fixed point. We conclude with a discussion of possible experimental realizations in iron-based superconductors.