Muonium (Mu), a bound state of a positively charged muon and an electron, can diffuse through crystal lattices and interact with defect centers in insulators and semiconductors. We demonstrate that this Mu’s diffusive property can be used to probe defects in a diamond crystal lattice, specifically, substitutional nitrogen atoms (${\mathrm{N}}_{\text{s}}^0$) and nitrogen-vacancy (NV) centers in type-Ib diamond. Upon interaction with these defects, Mu can exchange its electron’s spin or change its charge state, which results in muon spin relaxation. However, muons in diamond (and semiconductors in general) can be in a few distinctive muonium states, with each state contributing to the muon signal. In addition, these states can undergo site and charge exchange interaction, forming a dynamic network. Hence, to study the Mu interaction with point defects, the muon data have to be deconvoluted to isolate signals from the diffusing species. To achieve this goal, we have modeled the Mu state exchange dynamics and numerically simulated the time evolution of muon spin polarization by the density matrix method. With a global curve fit to a set of longitudinal field scan data, one can extract the Mu transition rates that involve interaction with the defect centers. The diffusing tetrahedral interstitial Mu was found to interact with the paramagnetic ${\mathrm{N}}_{\text{s}}^0$ center via electron spin exchange. In contrast, they are converted to form a diamagnetic center upon interaction with the negatively charged NV center.

The confinement of electromagnetic radiation within extremely small volumes offers an effective means to significantly enhance light-matter interactions, to the extent that zero-point quantum vacuum fluctuations can influence and control the properties of materials. Here, we develop a theoretical framework for the quantum electrodynamics of graphene Landau levels embedded in a deep-subwavelength hyperbolic cavity, where light is confined into ultrasmall mode volumes. By studying the spectrum, we discuss the emergence of polaritons, and disentangle the contributions of resonant quantum vacuum effects from those of purely electrostatic interactions. Finally, we study the hybridization between magnetoplasmons and the cavity modes.

The topology of ${\mathrm{WTe}}_2$, a transition-metal dichalcogenide with large spin-orbit interactions, is thought to combine type II Weyl semimetal and second-order topological insulator (SOTI) character. The SOTI character should endow ${\mathrm{WTe}}_2$ multilayer crystals with topologically protected helical states at their hinges, and, indeed, one-dimensional states have been detected thanks to Josephson interferometry. However, the immunity to backscattering conferred to those states by their helical nature has so far not been tested. To probe the topological protection of ${\mathrm{WTe}}_2$ edge states, we have fabricated superconducting quantum interference devices (SQUIDs) in which the supercurrent through a junction on the crystal edge interferes with the supercurrent through a junction in the bulk of the crystal. We find behaviors ranging from a symmetric SQUID to asymmetric SQUID patterns, including one in which the modulation by magnetic field reveals a sawtoothlike supercurrent versus phase relation for the edge junction, demonstrating that the supercurrent at the edge is carried by ballistic channels over 600 nm, a telltale sign of the SOTI character of multilayer ${\mathrm{WTe}}_2$.

The functional renormalization group (fRG) is acknowledged as a powerful tool in quantum many-body physics and beyond. On the technical side, conventional implementations of the fRG rely on regulators for bare propagators only. Starting from Schwinger-Dyson and Bethe-Salpeter equations, we develop here an fRG formulation where both bare propagators and bare interactions can be dressed with regulators. The approach thus obtained is an extension of the multiloop fRG recently introduced for many-fermion systems. Using the single-boson exchange decomposition, we show that the underlying flow equations are simply interpreted as adding a regulator to the bosonic propagator and that such an extension scarcely changes the original structure of the flow equations. Overall, we provide a framework for implementing approaches that cannot be realized with conventional fRG methods, such as temperature flows for models with retarded interactions. For concrete applications, we analyze the loop convergence of our formulation against a conventional cutoff scheme for the Anderson impurity model. Finally, we devise a temperature-flow scheme that implements a cutoff in both the propagator and the bare interaction, and demonstrate its validity on a model of an Anderson impurity coupled to a phonon.

Quantum heat engines are nanoscale devices that convert heat into work by exploiting quantum effects, such as coherence and interference. Previous studies of these devices did not consider spin-dependent effects, which can influence the thermoelectric performance of the engine. In this work, we study the thermoelectric behavior of a quantum heat engine based on an Aharonov-Bohm ring—a mesoscopic ring where electrons exhibit interference depending on the magnetic flux it encloses—incorporating Rashba spin-orbit interaction (SOI), which couples the electron's motion and spin. We find that Rashba SOI enhances the figure of merit $ZT$, a measure of the engine's conversion efficiency. Our results suggest that controlling spin-dependent interference could lead to improvements in the fabrication of efficient thermoelectric devices.

We investigated electron-nuclear spin entanglement in the paramagnetic ground state of the Ho-based cubic compound ${\mathrm{HoCo}}_2{\mathrm{Zn}}_{20}$. From analyses of magnetization and specific heat data, we determined the cubic crystalline electric field (CEF) parameters, the magnetic exchange constant, and the hyperfine coupling constant between the $4f$ magnetic moment and the ${}^{165}\mathrm{Ho}$ nuclear spin. Our results show that the ${\mathrm{\Gamma }}_5$ CEF ground state is split by the hyperfine coupling, with an energy width of 1.3 K at 0 T, and that the true paramagnetic ground state is a quasisextet arising primarily from entanglement between the $f$-electron effective spin $S=1$ and the ${}^{165}\mathrm{Ho}$ nuclear spin $I=7/2$. We further demonstrate that, depending on the CEF parameters, the paramagnetic ground state can switch to an electron-nuclear coupled dectet. These findings underscore the importance of accurately identifying the electron-nuclear level scheme for understanding the low-temperature properties of rare-earth compounds containing spin-active nuclei.

${\mathrm{UCd}}_{11}$ is an antiferromagnetic uranium intermetallic compound ($T_{\mathrm{N}}=5.3\mathrm{K}$) with enhanced electron mass and uranium-uranium spacings nearly twice the Hill limit, suggesting a weakly hybridized $5f$ electronic character. Various x-ray spectroscopy techniques indicate that uranium in ${\mathrm{UCd}}_{11}$ adopts the formal ${\mathrm{U}}^{3+}5f^3$ configuration, while core-level photoemission spectroscopy (PES) data of ${\mathrm{UCd}}_{11}$ reveal only a weak satellite feature, typically interpreted as a signature of itinerancy. In this work, we present density functional theory (DFT) combined with dynamical mean-field theory (DMFT) calculations of ${\mathrm{UCd}}_{11}$, using material-specific parameters tuned to reproduce valence-band PES spectra at different photon energies, thereby exploiting the energy dependence of photoionization cross sections. Our results demonstrate that ${\mathrm{UCd}}_{11}$ is a highly localized uranium $5f^3$ system. Furthermore, core-level spectra obtained from a $\mathrm{DFT}+\mathrm{DMFT}$ Anderson impurity model reveal that, contrary to common assumptions, the presence or absence of satellite structures is not a reliable indicator of strong correlations or itinerant $5f$ behavior.

We present a systematic density-functional theory study of the effects of strain on the structural and electronic properties in vanadium dioxide (${\mathrm{VO}}_2$), with particular emphasis on its effect on the relative stability of the metallic rutile and the insulating monoclinic M1 phases. We consider various strain conditions that can be related to epitaxial strain present in ${\mathrm{VO}}_2$ films grown on different lattice planes. Our calculations confirm the dominant role of $c$-axis strain, i.e., along the direction of the V-V dimerization in the M1 phase. Our analysis suggests that this effect stems primarily from the weakening of the lattice stiffness, with the hopping amplitude along the $c$ axis playing a minor role. We also confirm that, in strain scenarios that deform the basal plane, the $c$-axis strain still has a dominant effect on the phase stability.

We report a random singlet ground state in the $S=\frac{1}{2}$ Heisenberg pyrochlore antiferromagnet ${\mathrm{NaCdCu}}_2{\mathrm{F}}_7$. Cationic ${\mathrm{Na}}^{+}$/${\mathrm{Cd}}^{2+}$ disorder on the pyrochlore $A$ site generates a broad distribution of ${\mathrm{Cu}}^{2+}\text{–}{\mathrm{F}}^{-}\text{–}{\mathrm{Cu}}^{2+}$ exchange couplings, introducing intrinsic magnetic bond disorder. Despite strong antiferromagnetic interactions $({\theta }_{\mathrm{CW}}=-72\mathrm{K})$, no magnetic order or global spin freezing is observed in dc and ac susceptibility, specific heat, or ${}^{23}\mathrm{Na}$ nuclear magnetic resonance to 120 mK, with muon spin relaxation experiments confirming persistent spin dynamics to 58 mK. $T$-linear specific heat, a Curie-like susceptibility tail, and power-law scaling with data collapse in $\chi \left(T\right)$, $M\left(H\right)$, $C_{\mathrm{mag}}/T$, ${}^{23}\mathrm{Na}$ $(1/T_{1}T)$, and the muon spin polarization $P\left(t\right)$ reveal a disorder-driven network of random singlets and orphan spins. Scaling across multiple bulk and local probes is consistent with a broad distribution of exchange energies, $P\left[\mathcal{J}\right]\sim {\mathcal{J}}^{-\alpha }$. This behavior contrasts with previously studied $\mathrm{Na}{A}^{″}{B}_2{\mathrm{F}}_7$ pyrochlore fluorides, where magnetic bond disorder precipitates spin-glass freezing, underscoring the crucial role of strong $S=\frac{1}{2}$ quantum fluctuations in ${\mathrm{NaCdCu}}_2{\mathrm{F}}_7$.

Ferrotoroidic materials, which break both time- and space-reversal symmetries to enhance magnetoelectric responses, are of great interest for applications. ${\mathrm{Ba}}_6{\mathrm{Cr}}_2{\mathrm{S}}_{10}$, a recently synthesized quasi-one-dimensional ferrotoroidic candidate, combines polarization, magnetization, and toroidal moment, yet its electronic band structure, magnetic evolution, and phase transitions remain incompletely understood to date. Here, we investigate ${\mathrm{Ba}}_6{\mathrm{Cr}}_2{\mathrm{S}}_{10}$, using electrical transport, direct current magnetization, alternating current susceptibility, specific heat, and muon spin rotation ($\mu \mathrm{SR}$) measurements, along with various first-principles calculations. Electrical transport measurements and density functional theory confirm a narrow band gap ($\sim 0.707\mathrm{eV}$, experimentally; $\sim 0.680\mathrm{eV}$, theoretically). $\mu \mathrm{SR}$ measurements reveal that static magnetism dominates down to 2 K, with an ordered volume fraction of $\sim 89\%$ at this temperature. Two distinct transitions are identified: one around the Néel temperature $[T_{\mathrm{N}}=(8\pm 1)\mathrm{K}]$ and another corresponding to a phase transition near 30 K, as consistently indicated by our complementary experimental techniques. The absence of dynamic critical behaviors and the lack of a $\lambda$-type specific heat anomaly are consistent with a first-order-like transition at $T_{\mathrm{N}}$. Based on various complementary experimental results and theoretical calculations, we propose a hypothesis that the $\sim 30\mathrm{K}$ broad feature arises from the complex interplay between crystal structure distortion and magnetic coupling, which is potentially directly linked to ferrotoroidicity. As the first study integrating $\mu \mathrm{SR}$ with complementary experiments and first-principles calculations to explore ferrotoroidicity, this work provides key insights into the underlying physics of ${\mathrm{Ba}}_6{\mathrm{Cr}}_2{\mathrm{S}}_{10}$ and related compounds in this area.

Interfacing light with quantum systems is an integral part of quantum technology, with the most essential building block being single-photon emitters. Although various platforms exist, each with its individual strengths, molecular emitters boast a unique advantage—namely, the flexibility to tailor their design to fit the requirements of a specific task. However, the characteristics of the vast space of possible molecular configurations are challenging to understand and explore. Here, we present a theoretical and computational framework to initiate exploration of this vast potential by integrating database analysis with microscopic predictions. Using a model system of dibenzoterrylene in an anthracene host as benchmark, our approach identifies promising new candidates, among them a chiral molecular emitter. Future extensions of our approach integrated with machine learning routines hold the promise of ultimately unlocking the full potential of molecular quantum light-matter interfaces.

We study the emergence of the nonlocal Josephson effect in a system composed of three laterally coupled minimal Kitaev chains and exploit it to realize the nonlocal Josephson diode effect. We find that an imbalance between crossed Andreev reflections and electron cotunneling in the middle Kitaev chain gives rise to an asymmetric $2\pi$-periodic phase-dependent Andreev spectrum, controlled by the superconducting phases across the left and right junctions. We then show that the asymmetric Andreev spectrum, formed by hybridized Andreev bound states at the left and right junctions, enables a supercurrent across one junction via the phase difference at the other junction, thereby signaling the nonlocal Josephson effect. Notably, these nonlocal Josephson supercurrents exhibit distinct positive and negative critical currents, demonstrating the realization of the nonlocal Josephson diode effect with highly tunable polarity and efficiencies exceeding 50%. The nonlocal Josephson diode effect requires breaking local time-reversal and local charge-conjugation symmetries, with the latter being unique to minimal Kitaev chains. Our results establish minimal Kitaev chains as a highly controllable platform for engineering nonlocal Josephson phenomena.

We present the diagrammatic theory of the irreducible self-energy and Bethe-Salpeter kernel that naturally arises within the Green’s function formalism for a general $N$-body non-Hermitian interaction. In this work, we focus specifically on the coupled-cluster (CC) self-energy generated by the similarity transformation of the electronic structure Hamiltonian. The main result of this work is a Green’s function formalism for non-Hermitian many-body quantum systems. We develop the biorthogonal quantum theory to construct dynamical correlation functions where the time dependence of operators is governed by a non-Hermitian Hamiltonian. We extend the Gell-Mann and Low theorem to include non-Hermitian interactions and to generate perturbative expansions of many-body Green’s functions. We introduce the single-particle coupled-cluster Green’s function and derive the perturbative diagrammatic expansion for the non-Hermitian coupled-cluster self-energy in terms of the “noninteracting” reference Green’s function, $\tilde{\mathrm{\Sigma }}\left[G_0\right]$. From the exact equation of motion of the single-particle coupled-cluster Green’s function, we derive the self-consistent renormalized coupled-cluster self-energy, $\tilde{\mathrm{\Sigma }}\left[\tilde{G}\right]$, and demonstrate its relationship to the perturbative expansion of the self-energy, $\tilde{\mathrm{\Sigma }}\left[G_0\right]$. Subsequently, we show that the usual electronic self-energy can be recovered from the coupled-cluster self-energy by neglecting the effects of the similarity transformation. We show how the coupled-cluster ground-state energy can be obtained from the coupled-cluster self-energy and provide an overview of the relationship between approximations for the coupled-cluster self-energy, ionization potential/electron affinity equation-of-motion coupled-cluster theory, and the $G_0{W}_0$ approximation. As a result, we introduce the CC-$G_0{W}_0$ self-energy by leveraging the connections between Green’s function and coupled-cluster theory. Finally, we derive the diagrammatic expansion of the coupled-cluster Bethe-Salpeter kernel.

We develop a framework to describe collective buckling in metal-organic frameworks (MOFs). Starting from the microscopic structure of a single organic linker, we define a buckling coordinate governed by an effective double-well potential. Coupling between linkers is introduced within a dipole-dipole approximation, resulting in an effective lattice Hamiltonian. We analyze the transition between ordered and disordered phases within a mean-field approximation and estimate the critical temperature. As an illustrative example for our theory, we discuss the collective buckling instability for the prototypical cubic framework MOF-5 under different values of uniaxial strain. Our approach provides a quantitative description of collective buckling in framework materials.

Whether and how a system approaches equilibrium is a central issue in nonequilibrium statistical physics, underpinning our understanding of both thermalization and heat transport. Bogoliubov's three-stage (initial, kinetic, and hydrodynamic) evolution hypothesis provides a qualitative framework, but quantitative advances have mostly been restricted to near-integrable systems such as dilute gases. In this work, we investigate the relaxation dynamics of a one-dimensional diatomic hard-point gas and present a phase diagram that comprehensively characterizes relaxation behavior across the full parameter space, from near-integrable to far-from-integrable regimes. We first analyze thermalization (local energy relaxation in nonequilibrium states) and identify three universal dynamical regimes: (1) In the near-integrable regime, kinetic processes dominate (initial $+$ kinetic phase), local energy relaxation decays exponentially, and the thermalization time $\tau$ scales with perturbation strength $\delta$ as $\tau \propto {\delta }^{-2}$. (2) In the far-from-integrable regime, hydrodynamic effects dominate (initial $+$ hydrodynamic phase), local energy relaxation follows power-law decay, and thermalization time scales linearly with system size $N$. (3) In the intermediate regime, Bogoliubov phase emerges, marked by the transition from kinetic to hydrodynamic relaxation. The phase diagram further reveals that hydrodynamic behavior can emerge even in small systems when the system is sufficiently far from the integrable regime, challenging the conventional view that such effects are only significant in large systems. In the thermodynamic limit, the system's relaxation behavior depends on the order in which the limits ($N\to \infty$ or $\delta \to 0$) are taken. We then turn to heat transport (decay of heat-current fluctuations in equilibrium) and demonstrate that its behavior is fully consistent with the thermalization process, leading to the first unified theoretical description of thermalization and heat transport. Our results establish a comprehensive view of relaxation in classical many-body systems and suggest a route for studying similar dynamics in quantum systems, highlighting universal scaling laws and the interplay of kinetic and hydrodynamic processes across integrability regimes.

We report a real-time first-principles study of ultrafast spin injection in a ${\mathrm{WSe}}_2$-graphene heterobilayer under circularly polarized laser irradiation, using time-dependent density functional theory. Contrary to conventional expectations, spin transfer into graphene is not a passive process but is actively driven by spin-selective carrier filtering at the interface. Spin-polarized carriers generated in the ${\mathrm{WSe}}_2$ layer induce a preferential migration of opposite-spin carriers from graphene, which results in net spin magnetization in graphene. This process is governed by interlayer band offsets, density-of-state asymmetry, and Pauli blocking. These findings indicate a microscopic mechanism of spin injection in nonmagnetic systems and identify a guiding principle for the design of ultrafast optospintronic functionalities in van der Waals heterostructures.