We show that the quantum geometry of fractionalized spin excitations in Mott-insulating quantum spin liquids (QSLs) gives rise to a finite Raman circular dichroism (RCD) signal. We identify conditions under which the Loudon-Fleury framework, where light modulates the underlying exchange interactions, and the light-matter-coupling approach, where the effective vector potential couples directly to the spinon bands, yield equivalent Raman vertices for the fractionalized quasiparticles. Using the latter, we derive an analytical decomposition of the RCD into different quantum geometric contributions. This reveals the sensitivity of the RCD to the underlying structure of the wave functions and the handedness of the excitations, rather than a nonzero Chern number of spinon excitations. To illustrate this, we apply our approach to two examples, the Kitaev honeycomb model in a magnetic field and a parton mean-field state of the chiral QSL on the triangular lattice.

We theoretically study the non-Hermitian topology of magnons with finite lifetimes due to Gilbert damping. By incorporating the spin-wave theory and perturbation theory for the Landau-Lifshitz-Gilbert equation including nonlocal damping terms, we analytically evaluate the spectral winding number for point gaps, which indicates the existence of the non-Hermitian skin effect (NHSE). We find that, in noncollinear magnetic textures, the NHSE can occur even in the absence of nonlocal damping, due to the momentum-dependent mixing of the particle and hole components of magnon eigenmodes, which is absent in collinear magnets with a single magnon band. In the presence of nonlocal damping along one direction, we show that the winding number for an energy band with a unique minimum is determined from the sign of the wave number at the band minimum. We demonstrate these results using a model that hosts a skyrmion-string lattice as a steady state. We further investigate spin-wave propagation dynamics excited by a magnetic-field pulse and show that the propagation direction changes drastically from band to band depending on the presence of local and nonlocal damping, consistent with the nontrivial winding numbers.

We derive a microscopic theory of drag for a Bose–Einstein condensate interacting with a moving reservoir. Starting from interactions between a condensate and a drifting fermionic bath, we integrate out the bath degrees of freedom within the Born–Markov approximation to obtain an effective Gross–Pitaevskii equation with a drag potential. From this potential we derive an effective drag force and obtain a closed expression for the drag coefficient, determined by the reservoir density fluctuations and the condensate density profile. As an application, we simulate the drag for an exciton–polariton condensate interacting with a drifting electron gas.

The atomistic mechanisms governing solvent-mediated exfoliation of transition metal dichalcogenides remain poorly understood, limiting the rational design of scalable synthesis strategies. Here we investigate the role of hydrazine (${\mathrm{N}}_2{\mathrm{H}}_4$) in the liquid-phase exfoliation of ${\mathrm{TiSe}}_2$ and copper-intercalated ${\mathrm{TiSe}}_2$ (${\mathrm{Cu}}_x{\mathrm{TiSe}}_2$), is investigated combining density functional theory (DFT) calculations with experimental solvothermal synthesis under subcritical conditions. ${\mathrm{N}}_2{\mathrm{H}}_4$ is found to physisorb on pristine ${\mathrm{TiSe}}_2$ but chemisorb on ${\mathrm{Cu}}_x{\mathrm{TiSe}}_2$, driven by the formation of an N-Cu bond. This enhanced chemical affinity, rather than a simple geometric expansion of the interlayer spacing, is identified as the primary driver of the selective, copper-promoted exfoliation observed experimentally. ${\mathrm{N}}_2{\mathrm{H}}_4$ intercalation further increase the equilibrium interlayer separation and reduces the exfoliation energy. AFM characterization confirms the synthesis of monolayer ${\mathrm{Cu}}_x{\mathrm{TiSe}}_2$ sheets. These results reveal the key interactions that enable ${\mathrm{N}}_2{\mathrm{H}}_4$-assisted exfoliation of ${\mathrm{TiSe}}_2$ and its copper-intercalated analogs.

The low-temperature thermal properties of amorphous solids, from the tunneling regime to the boson peak (BP), are described in terms of the redistribution of low-energy excitations induced by structural disorder and the progressive loss of vibrational coherence. We develop an interacting two-level-system (ITLS) model based on a coarse-grained three-level Hamiltonian that provides an effective representation of two classes of low-energy excitations: localized tunneling excitations and higher-energy excitations arising from their coupling with elastic vibrations. From the corresponding equilibrium partition functions, we derive the specific heat and construct an effective thermodynamic spectral density that describes the temperature-dependent redistribution of spectral weight between localized and progressively hybridized excitations. Within this picture, the experimentally established excess of vibrational states remains fully consistent with the present description. The ITLS framework further resolves its thermodynamic signature into a low-energy soft contribution, which already develops a boson-peak-like maximum, and higher-energy hybridized contributions that are required to reproduce the complete experimental peak. A finite coherence length $\xi$ introduces an effective cutoff frequency ${\nu }_{\mathrm{off}}$ that delimits the thermodynamically active spectral window. Fits to specific-heat measurements for several glass-forming systems reproduce the experimental behavior over the temperature range considered and yield cutoff frequencies comparable to reported Ioffe–Regel crossover frequencies. Our results provide a unified equilibrium thermodynamic description of the tunneling regime and the boson peak and connect their spectral signatures with the progressive loss of vibrational coherence in disordered solids.

${\mathrm{Mg}}_2{\mathrm{IrH}}_6$ is a metastable complex metal hydride with a predicted superconducting transition temperature as high as $170\mathrm{K}$ at ambient pressure. Following the synthesis of isomorphic, insulating ${\mathrm{Mg}}_2{\mathrm{IrH}}_5$ at low pressure, higher-pressure studies were conducted to investigate the phase behavior and compound formation in this system. X-ray diffraction and Raman spectroscopic measurements, combined with ab initio calculations, support the formation of cubic ${\mathrm{Mg}}_2{\mathrm{IrH}}_7$ above ca. 40 GPa. This phase may coexist with a related hexagonal hydride with an estimated composition near ${\mathrm{Mg}}_2{\mathrm{IrH}}_5$. Electrical transport measurements suggest that cubic ${\mathrm{Mg}}_2{\mathrm{IrH}}_7$ is insulating, in agreement with ab initio predictions, and persists during room-temperature decompression until $\sim 20\mathrm{GPa}$ before reverting back to cubic ${\mathrm{Mg}}_2{\mathrm{IrH}}_5$. The experimental results support ground-state structure predictions in the Mg-Ir-H system, and the formation of two nearly identical phases with surrounding compositions may open opportunities to access superconducting ${\mathrm{Mg}}_2{\mathrm{IrH}}_6$ through nonequilibrium processing pathways.

Charge transport in superconducting junctions at finite voltages is governed by Andreev reflections, including multiple Andreev reflections, which are processes that enable multiple charge transfer, a hallmark that shot noise can directly quantify. Since the effective charge extracted from shot-noise measurements varies with the transparency of the junction, systematic control of transparency is essential but experimentally challenging. Here, we present shot-noise scanning tunneling microscopy measurements enabled by a newly developed amplifier, allowing access to different transparency regimes. We perform shot-noise measurements on Pb(111) with tunable transparency at 2.2 K and observe that the shot noise evolves from a single-electron tunneling regime to multiple charge-transfer regimes as transparency increases. Our results are quantitatively consistent with theoretical simulations of Andreev reflections and multiple Andreev reflections for a single-channel system. These results establish junction transparency as the key parameter governing the evolution of charge transport and demonstrate that noise STM is a powerful platform for investigating microscopic charge transport mechanisms with controlled junction transparency at the atomic scale.

Confinement produces mesonic bound states, but their existence alone does not determine the statistical organization of the many-body spectrum. We investigate this connection in the spin-$\frac{1}{2}$ XXZ chain subject to a longitudinal staggered field, using symmetry-resolved exact diagonalization for spin chains of up to $N=22$ sites. As the exchange anisotropy is increased into the antiferromagnetic Ising regime, the finite-size spectra cross over from Gaussian-orthogonal-ensemble-like statistics to reduced level repulsion. This evolution occurs in both the zero-momentum and generic nonzero-momentum sectors examined. Direct measurements of the domain-wall-number operator and its eigenstate variance reveal increasingly sharp approximate domain-wall sectors, accompanied by banding of spin correlations and bipartite entanglement. Statistics within a selected domain-wall band further characterize the spectral organization beyond the full-sector gap-ratio analysis. We identify the one-meson branch and compare its energies and level spacings with established Airy and semiclassical descriptions and a finite-chain strong-anisotropy treatment, quantifying their accuracy and systematic deviations. These results connect the known confined two-spinon spectrum to the broader finite-anisotropy organization of many-body eigenstates and provide quantitative evidence for emergent domain-wall constraints in the full microscopic Hamiltonian.

This work introduces a theoretical framework based on the Kadanoff-Baym equations in spinor space to study ultrafast magnetization dynamics in itinerant ferromagnetic systems from first principles. By incorporating spin-orbit coupling into the ab initio Hamiltonian and generalizing the self-energies to include terms beyond the charge sector, I derive scattering integrals within the Markov approximation and quasiparticle renormalizations for electrons and phonons in the presence of spin-dependent effective interactions within a many-body perturbation theory approach. I explicitly discuss how magnons emerge in this framework, and derive, through suitable approximations, a closed and tractable set of equations for coupled electron, phonon, and magnon dynamics that can be solved from first principles. I discuss the equations of motion within a minimal ferromagnetic model, where the coupled equations are solved numerically. This approach allows one to treat coherent and incoherent magnetic dynamics on an equal footing, paving the way for a first-principles understanding of ultrafast magnetic dynamics and demagnetization, and opening the door to a truly predictive theory of femtomagnetism in periodic systems.

Light-induced superconducting-like states have been reported in several classes of correlated materials. From a theoretical point of view, the induction of $\eta$-pairing is a promising route to nonthermal superconductivity. Numerical studies of photo-doped Mott systems revealed $\eta$-pairing states with very high effective critical temperatures. These investigations were, however, restricted to particle–hole symmetric states in large-gap Mott insulators, while the experiments were performed on strongly correlated metallic systems without particle-hole symmetry. It is thus relevant to explore whether $\eta$-pairing also exists in non-particle–hole symmetric setups and in photo-excited metallic states. Here, we use steady-state nonequilibrium dynamical mean-field theory combined with a strong-coupling impurity solver up to third order to investigate this issue. We find that in the strongly correlated regime with a large Mott gap, and for low effective doublon and holon temperatures, $\eta$-pairing is robust against changes in the total filling and an imbalance in the doublon and holon density. An asymmetry in the effective doublon and holon temperatures can, however, strongly suppress the order parameter. In photo-doped metallic systems with a three-peak structure in the local density of states, $\eta$-pairing can be realized in setups with positive doublon and holon temperatures and a population inversion in the low-energy quasiparticle band.

The tunability and control of topological edge/surface states are crucial for the development of new device applications. In this work, by combining first-principles calculations and Wannier-based tight-binding methods, we show the emergence of quantum spin Hall phases in van der Waals heterostructures formed by stacking two trivial quintuple layers from the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ family. We demonstrate the tunability of the edge states under interlayer strain and external electric field effects, suggesting the possibility of switching topological edge states on/off by external control. Additionally, the quantum spin Hall edge channels remain robust against interlayer twist, highlighting their stability against external perturbations. Our results provide an alternative way to create and manipulate two-dimensional topological phases in systems based on the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ family, which can be valuable for practical applications, such as topological field-effect transistors and spintronic devices.

We report the study of plasmon-exciton coupling in nanoparticles with a metal core and an outer J-aggregate shell. Our theoretical consideration shows that in many cases of practical importance, the use of the conventional model of two coupled oscillators containing a single coupling constant $g$ is oversimplified for a reliable description of plexcitonic (plasmon-exciton) coupling effects in such systems. Therefore, one needs to develop a more justified approach for accurate evaluation of the plexcitonic coupling efficiency. Here we demonstrate that for core-shell nanospheres, two plexcitonic coupling constants $g_{+}$ and $g_{-}$ emerge naturally in our theory because of the electromagnetic coupling of the dipolar localized surface plasmon-polariton with two excitonic shell resonances. Such resonances are caused by the splitting of a J-aggregate peak owing to the presence of inner and outer boundaries of the organic shell. Our approach is rigorously justified for hybrid core-shell nanoparticles of spherical shape, and it is more suitable for a reliable description of light extinction spectra and for quantitative determination of plexcitonic coupling efficiency than the model of two coupled oscillators. We derive simple analytical expressions for the introduced coupling constants, $g_{+}$ and $g_{-}$. Our calculations for various plexcitonic nanoparticles with a gold or silver core and an isotropic J-aggregate shell are in good agreement with the extinction spectra obtained using exact equations. This work broadens the understanding of the physical phenomena caused by plasmon-exciton coupling and provides a means for the correct determination of its efficiency.

We study an infinite-range coupled electronic-pseudospin and nuclear-spin model for ferroquadrupolar and nuclear-spin ordering in ${\mathrm{TmVO}}_4$ in external magnetic and strain fields. This material is an experimental realization of a transverse-field Ising model, where the Ising degree of freedom is quadrupolar and nonmagnetic, but a transverse component is magnetic and couples both to external magnetic fields and to the nuclear spins via a hyperfine coupling. At zero external magnetic field, the electronic and nuclear degrees of freedom undergo well-separated two-stage ordering, accompanied by the release of their respective entropies. A transverse magnetic field polarizes the electronic orbital moments and, via the hyperfine coupling, the nuclear spins. The quadrupolar ordering temperature is gradually reduced to zero. However, the nuclear transition is replaced by a crossover at nonzero magnetic field. Quantum fluctuations are enhanced near the phase transitions and lead to peaks in the magnetic susceptibility. The spectral functions reveal a softening of a low-energy mode near the quantum critical point, consistent with the closing of the excitation gap and its reopening in the disordered phase. These features provide direct dynamical signatures of the field-driven quantum critical phenomena.

The density functional theory plus Hubbard $U$ ($\mathrm{DFT}+U$) technique is one of the most widely used tools by condensed matter physicists and solid-state chemists for the simulation of transition-metal and lanthanide-bearing crystals, and increasingly of much more diverse chemistries. Although often synonymous with the corrective functionals of Dudarev et al. [Phys. Rev. B 57, 1505 (1998)] and Liechtenstein et al. [Phys. Rev. B 52, R5467 (1995)], there exists a wide variety of $\mathrm{DFT}+U$-type functionals ready to be utilized, and no doubt yet to be developed. Since the earliest days, the gap in the $\mathrm{DFT}+U$ single-particle eigenspectrum has been associated with the fundamental band gap, and the method has typically found more success for spectra than for total-energy derived properties. There has been some doubt, however, as to the conceptual validity of this association. Here, extending findings from recent years regarding local, semilocal, and hybrid functionals within the generalized Kohn-Sham framework, we prove and numerically demonstrate that the $\mathrm{DFT}+U$ eigenspectrum gap is conceptually valid, in the specific sense that it matches its own fundamental gap calculated using total-energy differences. We emphasize that this does not imply its agreement with experimental values, and indeed our argument is independent of the Hubbard $U$ parameter. The result holds for pristine periodic systems with converged $k$-point sampling but not, however, for defective ones, isolated systems, or systems in which added charges exhibit spontaneous localization. We show that band-gap validity for pristine solids holds in the presence of pseudopotentials and PAW potentials, when using hybrid functionals, and in DFT+$U(+J)$ irrespective of the level of subspace projection onto the band-edge states. We survey every collinear-spin $\mathrm{DFT}+U$-type functional known to have been published to date, within a unified notation. We verify analytically under which conditions the eigenvalue gap equals its fundamental gap, for each functional. Returning to the related but different question of band-gap correction efficiency, we offer fresh analysis of $\mathrm{DFT}+U$ band-gap projection dependence, and each functional's effect on energies and gaps for the hydrogen lattice in the Mott-Hubbard limit.

We develop a computationally efficient finite-temperature continuum approach for exciton renormalization and optical absorption in doped monolayer transition-metal dichalcogenides. The model combines a massive-Dirac electronic structure, the nonlocal Rytova-Keldysh interaction, finite-temperature carrier screening, Hartree-Fock (HF) and random phase approximation (RPA) quasiparticle self-energy corrections, and a momentum-space Bethe-Salpeter equation (BSE). This approach enables the quasiparticle gap, exciton transition energies, and exciton binding energies to be followed systematically as functions of carrier density, temperature, dielectric environment, material parameters, and excitonic state. We find that quasiparticle BGR and binding-energy reduction can substantially compensate, producing comparatively weak shifts of the lowest exciton resonance, while excited states show a stronger response to screening. The RPA calculations further reveal a nonmonotonic temperature dependence of the excitonic energies at low carrier densities, reflecting the competition between quasiparticle and excitonic renormalizations. The calculated optical spectra also exhibit pronounced material-dependent density and temperature trends. These results substantially advance the compensation picture, providing a unified finite-temperature framework for excitonic many-body renormalization in monolayer TMDCs.

The discovery of high-$T_c$ superconductivity in pressurized bilayer nickelate ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ presents a new arena for exploring unconventional pairing mechanisms. A pivotal yet unresolved issue is the specific role of the $3d_{z^2}$ orbital of Ni. While its interlayer superexchange antiferromagnetic coupling is widely considered crucial for superconductivity, the role of its itinerancy remains undetermined. Early studies showed that the superconductivity is accompanied by the emergence of a small Fermi pocket of $3d_{z^2}$ orbitals. However, recent experiments show controversial results on the role of the $3d_{z^2}$ Fermi pocket on superconductivity. Motivated by these experimental results, we investigate an effective bilayer two-orbital model for ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ using density-matrix renormalization group on a minimal one-dimensional geometry. By systematically varying the $3d_{z^2}$ orbital filling from $1/12$ doping to half filling, we observe a pronounced suppression of superconducting correlations near half filling. Our results demonstrate the itinerancy of the $3d_{z^2}$ orbital is favorable for pairing in the bilayer two-orbital model for ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$. Moreover, we observe that the pairing correlation is enhanced in regions where charge fluctuations are large, suggesting a competition between charge order and superconductivity in the model.

The interplay between long-range structural distortions and orbital order, and local short-range phenomena such as polarons, are key for understanding the properties of the manganites. In this work we examine the role of local and long-range physics in relation to orbital ordering in the single-layered manganite ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{1.5}{\mathrm{MnO}}_4$ with a combination of optical reflection anisotropy and ultrafast broadband pump-probe spectroscopy. We find that the reflection anisotropy, measured in equilibrium, is strongly sensitive to charge and orbital-ordering transition only. However, the ultrafast response, measuring the nonequilibrium state, is sensitive to a range of phenomena in the material, including delocalized (long wavelength) phonons as well as localized polarons. In particular, we find that a strong sensitivity to electronic and structural changes gives rise to apparent nonlinearities when probed at specific wavelengths, despite the key degrees of freedom remaining linear. These observations point to the important role of orbital-lattice couplings and polarons dressing the electronic states across the ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{1.5}{\mathrm{MnO}}_4$ phase diagram.

In gyrotropic crystals, an oscillating magnetic field induces a charge response known as the gyrotropic magnetic current. While its conventional origin is attributed to magnetic-field-modified band energy and shifts in the Fermi surface, a recent study identified an additional spin-driven magnetic displacement contribution. Here, we complete the picture by identifying the orbital counterpart of the magnetic displacement current. Using a density matrix formulation that incorporates both minimal coupling and spin-Zeeman interactions, we derive the electronic equations of motion in the presence of an oscillating magnetic field and uncover an orbital contribution to the wave-packet velocity. Physically, this contribution arises from the time variation of the magnetic-field-induced charge polarization. In the low-frequency transport regime, this mechanism becomes purely intrinsic. We illustrate this intrinsic gyrotropic current of orbital origin in the $\mathcal{P}\mathcal{T}$-symmetric antiferromagnet CuMnAs. We show that the intrinsic gyrotropic magnetic current reverses sign upon Néel vector reversal, establishing it as a direct probe of antiferromagnetic order in CuMnAs and other $\mathcal{PT}$-symmetric antiferromagnets.

Peculiar features of the Josephson effect in graphene were described theoretically by Titov and Beenakker [Phys. Rev. B 74, 041401(R) (2006)], who solved the Dirac-Bogoliubov-de Gennes equation for a superconductor-graphene-superconductor junction with rectangular geometry. Here, we adopt the analysis for graphene Corbino disks, finding out that—for the outer to inner radii ratio $r_2/r_1\gtrsim 5$—such systems may demonstrate, when varying the electrochemical potential and the spatial profile of the electrostatic barrier, crossover from standard Josephson tunneling (SJT), via graphene-specific multimode Dirac-Josephson tunneling (MDJT), toward the ballistic Josephson effect (BJE). Signatures of SJT appear only near the Dirac point when the barrier shape is close to rectangular, MDJT appears in the tripolar range and is very robust against varying the barrier shape, and BJE is restored in the unipolar range when smoothing the barrier shape. A comparison with the results of a numerical simulation of quantum transport on the honeycomb lattice is also given.

We consider the renormalization of transport in a quantum contact by the external electromagnetic environment and show that weak quantum fluctuations from a symmetric environment can lead to a quantized transconductance in five-terminal contacts. This mechanism does not work for fewer terminals. We present an implementation of the environment and investigate several example quantum contacts where the quantization takes place.