The organic conductor $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ provides the prime example of a charge-order-driven metal-insulator transition. Restricted chemical substitution of S atoms by Se in the constituent molecules allows us to modify the electronic properties. This not only decreases the transition temperature but, in addition, alters the phase transition mechanism, resulting in the ground state deviating from the charge-ordered insulator state of the parent compound. Employing infrared optical spectroscopy, we investigate changes in the charge dynamics. Furthermore, we demonstrate the absence of charge ordering in the Se-substituted materials and suggest that the phase transition is instead driven by the localization of the itinerant charge carriers due to strong electron-phonon interactions.

In interacting topological systems, Landau-like order parameters interplay with the band topology of fermions. The physics of domain formation in such systems can get significantly altered due to the presence of topological fermions. In this work, we show that coupling a topological fermionic field to a scalar field can drastically modify the latter's nucleation processes. We find that the existence of nontrivial fermionic boundary modes on the nucleating droplets of the scalar field leads to substantial quantum corrections to the interface energy. This leads to an increase in the size of the critical nucleus beyond which unrestricted droplet growth happens. To illustrate the phenomena, we devise a minimal model of fermions in a Chern insulating system coupled to a classical Ising field in two spatial dimensions. Using a combination of analytic and numerical methods, we conclusively demonstrate that topological phases have a characteristic quantum correction to the interfacial line energy. Apart from material systems, our work opens up a host of questions regarding the impact of fermionic topological terms on classical phase transitions and associated criticality.

We investigate a minimal model of a two-terminal Josephson junction with conventional superconducting (SC) leads and a pair of interconnected quantum dots in the presence of two Aharonov-Casher (AC) fluxes. The Andreev bound-state spectrum features Weyl nodes within a three-dimensional synthetic Brillouin zone defined in the space of these AC fluxes and the SC phase difference. The aim is to determine the location and topological charge of these nodes by probing the Berry curvature on closed surfaces that may enclose them. This is achieved by adiabatically varying the superconducting phase difference and AC fluxes along a path on these surfaces and measuring the associated currents. We define the kinematic curvature as the cross product of a tangent vector along the path and the vector of these currents. In the adiabatic regime, the path-averaged kinematic curvature provides a quantized response equal to the topological charge enclosed by the surface, provided the path uniformly and densely covers it.

Third-harmonic generation (THG) is a key nonlinear optical process for ultrafast imaging, terahertz signal generation, and symmetry-sensitive probes, often dominating in centrosymmetric materials where lower-order responses vanish. However, the role of band geometry, Fermi surface effects, and disorder in enabling large and tunable THG remains poorly understood. Here, we develop a finite-frequency quantum kinetic theory of THG based on the density matrix formalism, deriving the third-harmonic conductivity tensor. Our framework isolates five distinct band-geometric contributions to interband and intraband processes, separates Fermi sea from Fermi surface terms, and incorporates disorder effects phenomenologically. We further provide a complete symmetry classification of THG for all 122 magnetic point groups. Applying the theory to graphene and the spin-split altermagnet ${\mathrm{RuO}}_2$, we trace its THG response to specific geometric terms. These results establish a predictive foundation for designing materials with enhanced and tunable THG in the finite-frequency regime.

Dynamical response theory is employed to investigate the effects of various transverse confinements on electron correlations in the ground state of a ferromagnetic one-dimensional quantum wire for different wire widths $b$ and density parameters $r_{\text{s}}$. In the regime of a thin quantum wire, electrons are treated as a one-dimensional gas under different confinement models via effective electron-electron interaction potentials. Using the first-order random phase approximation (FRPA) including self-energy and exchange contribution, which provides the ground-state structure beyond the random phase approximation, we numerically compute the structure factor, pair-correlation function, correlation energy, and ground-state energy for various values of $b$ and $r_{\text{s}}$. Our results reveal that the correlation energy depends on the choice of confinement model. For the ultrathin wire $(b\to 0)$ in the high-density limit, we find that the correlation energy for transverse confinement models $V_1\left(q\right)$ (harmonic), $V_2\left(q\right)$ (cylindrical), and $V_5\left(q\right)$ (harmonic-delta) approaches ${\epsilon }_{\text{c}}\left(r_{\text{s}}\right)=-{\pi }^2/360\sim -0.02741$ a.u., which agrees with the exact results in this limit [P.-F. Loos, J. Chem. Phys. 138, 064108 (2013); V. Ashokan, et al., Phys. Rev. B 101, 075130 (2020)]. This clearly illustrates that for at least these three confinement potentials, the one-dimensional Coulomb potential can be regularized at interparticle distance $x=0$ to yield the same correlation energy. In contrast, other confinement potentials, $V_3\left(q\right)$ (infinite square well), $V_4\left(q\right)$ (infinite square-infinite triangular well), and $V_6\left(q\right)$ (infinite square-delta well), do not approach the same high-density limit; instead, the correlation energy tends to ${\epsilon }_{\text{c}}\sim -0.03002$ a.u. for these potentials. The percentage difference in correlation energy between the confinement models $V_1\left(q\right), V_2\left(q\right), V_5\left(q\right)$ and $V_3\left(q\right), V_4\left(q\right), V_6\left(q\right)$ is within about $10\%$ in the high-density limit. The ground-state properties obtained from the FRPA are compared with the available quantum Monte Carlo results in the high-density regime. We observe that the peak height in the static structure factor at $k=2k_{\text{F}}$ depends significantly on the confinement model. These prominent peaks at $k=2k_{\text{F}}$ are fitted with a function based on our finite wire-width theory, guided by insights from bosonization, demonstrating good agreement with our FRPA theory.

Janus structures with a built-in electric field have shown many exotic properties, especially for new mechanisms in water splitting and out-of-plane piezoelectricity. In this work, Janus 1T- and 2H-$M\text{SH}$ monolayers are designed, guided by the synthesized Janus 2H-MoSH monolayer. Among them, semiconducting Janus 1T- and 2H-$M\text{SH}$ ($M$ = Sc, Y, V, Nb, Ta) monolayers are first determined by first-principles and high-throughput calculations. Intriguingly, they emerge as promising photocatalysts with suitable band edge alignments for water splitting, owing to their intrinsic built-in electric field. In particular, 2H-TaSH shows strong light-harvesting ability in the infrared and visible regions, adequate overpotentials, high solar-to-hydrogen energy conversion efficiency and autonomous hydrogen and oxygen evolution. Furthermore, Janus 1T- and 2H-ScSH exhibit particularly large piezoelectric coefficients of $d_{31}$ = 3.93 and 3.10 pm/V, respectively, which are attributed to their exceptionally low Young's modulus combined with significant charge asymmetry between the two surfaces. Our work significantly expands the family of Janus transition metal sulfhydrates while simultaneously identifying promising candidate materials for both photocatalytic water splitting and piezoelectric applications.

Altermagnets have garnered great interest due to their nonrelativistic spin splitting and unique physical properties. However, the control of altermagnetic states remains underexplored. Here, we propose a multiferroic state, i.e., ferroelastic altermagnetic state, in which ferroelastic strain couples directly to the spin splitting. Through symmetry analysis and first-principles calculations, we identify the ferroelastic $d$-wave altermagnetism of puckered pentagonal ${\mathrm{CoSe}}_2$ monolayer. Interestingly, uniaxial stress can induce a ferroelastic phase transition, accompanied by a ${90}^{\circ }$ rotation of the spin-splitting bands. Cooperative rotation of the lattice and Néel vectors preserves the sign of Kerr angle, whereas noncooperative rotation reverses it. Our work provides a general strategy for manipulating altermagnetism in multiferroic systems and opens other avenues for exploring emergent magnetoelastic phenomena.

We report an ab initio investigation of functionalized and $3d$-electron-doped ${\mathrm{Cr}}_2\mathrm{C}$ MXenes. Upon functionalization, the ${\mathrm{Cr}}_2\mathrm{C}$ becomes chemically, dynamically, and mechanically stable, and it exhibits magnetic semiconducting behavior. ${\mathrm{Cr}}_2{\mathrm{CF}}_2$ stands out as a wide band gap semiconductor, possessing superexchange interaction mediated by F atoms within the layer; however, the applied strain transforms it from an indirect to a direct band gap semiconductor. Strong spin-phonon coupling found in ${\mathrm{Cr}}_2{\mathrm{CH}}_2$ is supported by the distorted Cr spin density due to the hydrogen environment. Two magnon branches, associated with two sublattice spins, are found in the ferromagnetic ${\mathrm{Cr}}_2{\mathrm{CO}}_2$ and antiferromagnetic ${\mathrm{Cr}}_2{\mathrm{CF}}_2$. Depending on the types of $3d$-electron dopants and functionalization, ${\mathrm{Cr}}_2\mathrm{C}$ MXenes (except for ${\mathrm{Cr}}_2{\mathrm{CO}}_2$) change from an indirect band gap magnetic semiconductor to different states of electronic and magnetic matter, including an exotic direct band gap magnetic bipolar semiconductor. In addition, we reveal a band inversion between the two highest valence bands in the Fe-doped ${\mathrm{Cr}}_2{\mathrm{CCl}}_2$.

The pursuit of high-performance, rare-earth-free magnetostrictive materials is crucial for advancing technologies in sensing, actuation, and microelectromechanical systems. Heusler alloys represent a promising, yet underexplored, class of materials for this purpose. In this work, we perform a systematic first-principles investigation of the magnetostrictive properties of 25 Co-based full Heusler alloys, ${\mathrm{Co}}_2\mathrm{YZ}$ (Y = V, Cr, Mn, Fe, Co; Z = Al, Ga, Si, Ge, Sn). Our screening identifies ten compounds with large predicted magnetostriction ($|{\lambda }_{001}|>100$ ppm), highlighted by ${\mathrm{Co}}_3\mathrm{Si}$ with a giant value of -966 ppm. Furthermore, we demonstrate two effective strategies for engineering magnetostriction: (i) tuning the Fermi level, which enhances the magnetostriction of ${\mathrm{Co}}_3\mathrm{Sn}$ to -905 ppm via Sb doping and (ii) amplifying the spin-orbit coupling, which boosts the magnetostriction of ${\mathrm{Co}}_2\mathrm{CrGa}$ to a colossal -1008 ppm through Re substitution. Our analysis reveals a general predictive rule, uncovering a linear relationship between the magnetostriction and the choice of the $Y$-site transition metal. This work not only identifies novel candidates for magnetostrictive applications, but also establishes clear, physically grounded design principles to accelerate the discovery of new functional magnetic materials.

Nonreciprocity of light emission, when the radiation intensity differs for two opposite propagation directions, is a rare phenomenon in solids because it requires a violation of the crystal symmetry with respect to time-reversal. Such a violation via time-reversal symmetry breaking can occur either due to an applied magnetic field or due to a magnetic ordering. We perform a detailed theoretical and experimental study of the photoluminescence (PL) nonreciprocity in the noncentrosymmetric tetragonal antiferromagnet ${\mathrm{CuB}}_2{\mathrm{O}}_4$, where this effect reaches 80% below the Néel phase transition temperature of $T_N=20\mathrm{K}$. The effect is observed for three sets of extremely narrow exciton and exciton-magnon PL lines, associated with Frenkel excitons on the ${\mathrm{Cu}}^{2+}$ ions in the magnetic $4b$ subsystem. A strong manifestation of the nonreciprocity of emission is found in certain geometries for the commensurate antiferromagnetic phase, as well as in other phases with incommensurate spin ordering. In accordance with the magnetic symmetry of ${\mathrm{CuB}}_2{\mathrm{O}}_4$, the nonreciprocity of emission is observed for light propagation along certain directions within the easy (001) plane. A rigorous quantum-mechanical analysis of the wave functions of the initial and final states of the ${\mathrm{Cu}}^{2+}$ ions responsible for the PL is performed for various experimental geometries of the crystallographic axes and the applied magnetic field. The analysis confirms that the nonreciprocity of emission from Frenkel excitons in ${\mathrm{CuB}}_2{\mathrm{O}}_4$ is due to the interference of magnetic-dipole and electric-dipole transitions of antiferromagnetically ordered $4b$ spins of the ${\mathrm{Cu}}^{2+}$ ions, in good agreement with the experimental data.

${\mathrm{Ca}}_3{\mathrm{Co}}_{2-x}{\mathrm{Mn}}_x{\mathrm{O}}_6$ is a quasi-one-dimensional Ising chain magnet with a unique collinear up-up-down-down $(\uparrow \uparrow \downarrow \downarrow )$ magnetic ordering, that gives rise to ferroelectricity below the Néel temperature of 13 K. This unique long-range magnetic ordering, however, was argued to get stabilized only with Co excess (i.e., $x<1.0$) nonstoichiometric ${\mathrm{Ca}}_3{\mathrm{CoMnO}}_6$, and not so for the chemically stoichiometric compound $x=1.0$. Combining several experiments along with first-principles density-functional-based calculations, we elucidate that this unique $\uparrow \uparrow \downarrow \downarrow$ magnetic ordering can be stabilized even in the stoichiometric ${\mathrm{Ca}}_3{\mathrm{CoMnO}}_6$ (i.e., $x=1.0$) compound, albeit through the presence of optimum cationic positional disorder, where some Mn and Co ions occupy the trigonal-prismatic (Co sites for the ordered case) and octahedral (Mn-sites for the ordered one) sites respectively through relative tuning of the various magnetic exchange interactions. Specifically, the relative energy stability of the $\uparrow \uparrow \downarrow \downarrow$ magnetic order in ${\mathrm{Ca}}_3{\mathrm{CoMnO}}_6$ exhibits a nonmonotonic dependency on the extent of cationic disorder with a maximum of around 16% disorder. Thus our study helps to elucidate the mechanism that leads to stabilization of this unique functional $\uparrow \uparrow \downarrow \downarrow$ magnetic ordering in this promising class of multiferroic compounds.

Recently, the bilayer perovskite nickelate ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ has been reported to exhibit high-temperature superconductivity near 80 K under a moderate pressure of about 14 GPa. To investigate the underlying pairing mechanism and symmetry in this complex system, we propose and analyze a mixed spin-1 and spin-$\frac{1}{2}$ bilayer $t\text{−}J$ model in the strong-coupling regime. This model explicitly incorporates the crucial role of strong Hund's coupling, which favors the formation of local spin-triplet states from the two on-site $E_g$ orbital electrons at half-filling. We further investigate the model using both slave-particle mean-field theory and the density matrix renormalization-group method. Our simulation results reveal that the dominant pairing channel is the interlayer one in the $3d_{x^2-y^2}$ orbital. Hund's coupling is shown to enhance superconductivity within a reasonable physical range. Moreover, electron doping strengthens superconductivity by increasing carrier density; in contrast, hole doping weakens superconductivity. These findings offer critical insights into the unconventional superconductivity of pressurized ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ and underline the important role of orbital-selective behavior and Hund's rule.

Near a two-dimensional Ising-type nematic quantum critical point, the quantum fluctuations of the nematic order parameter are coupled to the electrons, leading to non-Fermi-liquid behavior and unconventional superconductivity. The interplay between these two effects has been extensively studied through the Eliashberg equations for the superconducting gap. However, previous studies often relied on various approximations that may introduce uncertainties in the results. Here, we revisit the issue of how the superconducting transition temperature $T_c$ is affected by removing certain common approximations. We numerically solve the self-consistent Dyson-Schwinger equations of the electron propagator $G\left(p\right)$, the nematic propagator $D\left(q\right)$, and the vertex function ${\mathrm{\Gamma }}_{\mathrm{v}}^{1\mathrm{L}}(p+q,p)$ expanded up to the triangle order, without introducing further approximations. Our calculations reveal that the extended $s$-wave superconducting gap is the only convergent solution to the nonlinear gap equations. We investigate the evolution of $T_c$ as the system approaches the nematic quantum critical point from the disordered (tetragonal) phase. Under the bare vertex approximation, $T_c$ is monotonically enhanced. However, when vertex corrections are incorporated, $T_c$ initially increases but then decreases, with the maximum value of $T_c$ occurring at a point away from the quantum critical point. The obtained gap symmetry and the nonmonotonic behavior of $T_c$ are compared with recent experiments on doped FeSe materials.

Using Migdal-Eliashberg theory implemented in Electron Phonon Wannier (EPW) code, we have investigated anisotropic superconductivity in a ternary boride $\mathrm{Ta}{\left(\mathrm{MoB}\right)}_2$. It is a single-gap, anisotropic, phonon-mediated superconductor having a critical temperature ${\mathrm{T}}_{\mathrm{c}}\sim 19.3\mathrm{K}$. A dominant contribution to superconductivity arises from the robust coupling between electronic states, primarily created by the $d_{xy},d_{x^2-y^2}$ orbitals of Mo atoms and the in-plane vibrations of Mo atoms. A weak Fermi surface nesting and a small electron-phonon coupling cannot induce charge density wavelike instabilities, as evidenced by the lack of a significant peak in the real part of the total Lindhard susceptibility and the absence of phonon softening. Furthermore, we have studied its electronic and superconducting properties under hydrostatic pressure up to 76.69 GPa, owing to its low bulk modulus and metastability. The persistent reduction in the density of states at the Fermi level, Fermi surface nesting and the stiffening of phonon modes lead to a diminution of superconductivity under pressure up to 59.71 GPa. At 76.69 GPa, a modification in the topology of the Fermi surface, namely a Lifshitz transition, occurs resulting in a sudden enhancement of nesting. This enhanced nesting, in turn, induces an abrupt stabilization of superconductivity at 76.69 GPa, resulting in a V-shaped response to pressure.

Spin-wave soft modes are fundamental indicators of instabilities and phase transitions in magnetic systems. Here the role of soft modes in the nucleation of stripelike domains in synthetic antiferromagnetic multilayers is theoretically studied. Using a dynamic matrix formalism and micromagnetic simulations, the phase diagrams for the parallel, antiparallel, and spin-flop states are determined, along with the critical anisotropy fields that facilitate the formation of stripe domains. It is demonstrated that, in the parallel configuration, stripelike domains are predicted by softening at both positive and negative critical wave vectors ($k_{\mathrm{c}}$), whereas, in the antiparallel configuration, the stripe-domain instability is direction selective, with softening at only one propagation direction (nonreciprocal). Furthermore, stripe domains are predicted to form in multilayers with structural asymmetry (thickness variation in an outer layer), which breaks the $\pm k_{\mathrm{c}}$ degeneracy of soft modes and enforces single-$k_{\mathrm{c}}$ selection. The predicted stripe periods ${\lambda }_{\mathrm{c}}=2\pi /k_{\mathrm{c}}$ (90–250 nm) exhibit strong dependence on anisotropy strength and layer number. Therefore, spin waves provide a predictive and quantitative route to delineate phase boundaries and texture periods from soft modes, offering a versatile framework for engineering stripe-domain states in synthetic antiferromagnets and providing new opportunities for spintronic and magnonic applications.

Magnetic materials with three-dimensional (3D) frustrated spin networks manifest a variety of exotic quantum ground states. We investigate the magnetic and thermal properties and the electronic structure of ${\mathrm{Ba}}_2{\mathrm{YbTaO}}_6$, which has a 3D frustrated face-centered-cubic (fcc) lattice of $J_{\mathrm{eff}}=\frac{1}{2}$ moments. The low-temperature magnetic susceptibility confirms the $J_{\mathrm{eff}}=\frac{1}{2}$ state interacting antiferromagnetically with the Curie-Weiss temperature ${\theta }_{\mathrm{cw}}=-0.4\mathrm{K}$. The zero-field specific heat data reveal the absence of magnetic long-range order down to 100 mK. A significant difference in the magnetic entropy between the zero field and the applied magnetic field highlights its potential towards adiabatic demagnetization refrigeration applications. First-principles calculations reveal a combination of strong spin-orbit coupling and pronounced magnetocrystalline anisotropy. Realizing a spin-orbit-coupled $J_{\mathrm{eff}}=\frac{1}{2}$ system on an fcc lattice, ${\mathrm{Ba}}_2{\mathrm{YbTaO}}_6$ emerges as a compelling system to study the interplay of frustration, single-ion anisotropy, and quantum magnetism in 3D lattices.

In this work, we consider an instantaneous transition of a uniaxial dielectric formed by an array of short conducting wires (split-wire medium) into a usual wire medium (WM) of continuous infinitely long wires. Due to strong spatial dispersion in the WM, the known theory by Morgecnthaler of temporal discontinuities in continuous media cannot be applied, and we solve this problem in the time domain using temporal boundary conditions that we specially derive. We find that a wave with transverse magnetic polarization propagating in the split WM (before transition) transforms into four waves: a pair of transverse magnetic waves and a pair of transverse electromagnetic waves. The power flow splits so that one of the new propagation directions is along the wires. The analytical model is validated by full-wave simulations.

The Mott transition, a metal-insulator transition due to strong electronic interaction, is observed in many materials without an accompanying change of system symmetry. An important open question in Mott’s proposal is the role of long-range screening, whose drastic change across the quantum phase transition may self-consistently make the transition more abrupt, toward a first-order one. Here, we investigate this effect in a model system of hydrogen atoms in a cubic lattice, using charge self-consistent dynamical mean-field theory that incorporates approximately the long-range interaction within the density functional treatment. We found that the system is well within the charge-transfer (CT) regime and that the CT gap intimately related to the Mott transition closes smoothly instead. This indicates that the long-range screening does not play an essential role in this prototypical example. This finding can be understood from the fact that the obtained insulating phase in this model system is driven by strong local interaction, and the transition is associated with the closing of CT gap. Contrary to Mott’s length-scale argument, such energetic competition between kinetic energy and local interaction is thus insensitive to long-range screening.