In this work, we study the magnetization dynamics in a ferromagnet/insulator/ferromagnet trilayer sandwiched between two superconductors (S/F/I/F/S heterostructure). It is well-known that a conceptually similar S/F/S system is a platform for implementing ultrastrong magnon-photon coupling. Here, we demonstrate that in such a S/F/I/F/S heterostructure, ultrastrong magnon-magnon coupling between the two F layers also appears. The strength of this interaction is many times greater than the strength of the usual dipole-dipole interaction. It is mediated via Meissner currents excited in the superconductor layers by the magnon stray fields. The strength of the magnon-magnon coupling is anisotropic, and its anisotropy is opposite to the anisotropy of the magnon-photon coupling, which allows them to be separated. Both couplings become much stronger when the temperature drops below the critical temperature of the superconductor layers. This effect enables the implementation of an efficient tuning of the wave number in the S/F/I/F/S heterostructures controlled by temperature in a wide range of frequencies. Overall, the rich and tunable spectrum of S/F/I/F/S multilayers opens broad prospects for their application in magnonics.

Motivated by significant discrepancies between experimental observations of electron-doped cuprates and numerical results of the Hubbard model, we investigate the role of nearest-neighbor (NN) electron interactions $V$ by studying the $t-U-V$ model on square lattices. Upon doping $\delta =0.153$, by using constrained path quantum Monte Carlo method, we find that NN electron attraction $V$ can notably drive an exotic $p$-wave spin-triplet pairing, while the NN electron repulsion $V$ will suppress the $d_{x^2-y^2}$-wave ($d$-wave) pairing and triggers the $d_{xy}$-wave pairing. Especially in the intermediate coupling regime, as NN repulsion increases, the intensity of $d_{xy}$-wave pairing also increases, further suppressing the presence of $d$-wave pairing, which may help explain the notable suppression of $d$-wave pairing in electron-doped cuprate superconductors. Besides the pairing phase, we also find that the NN electron attraction $V$ has no significant effect on spin density wave (SDW) and charge density wave (CDW), but repulsion $V$ significantly enhanced CDW and suppressed SDW. Our study suggests the $t-U-V$ Hubbard model can serve as the minimal model to capture the essential physics of the electron-doped cuprates.

Low-energy lattice models are the cornerstone for studying many-body physics and interactions between the system and measurement fields. A key challenge is identifying appropriate quasiparticle states that canonically transform between momentum and real space while retaining the correlation, entanglement, and geometric properties—generally called the Wannier obstruction. Here, we introduce a phase-space approach to bypass these obstructions. Instead of treating the phase space as a manifold, we embed a real space through a Bloch vector space at each momentum. Orbital and spin states are introduced through product states with the Bloch vector, while quantum statistics, correlations, topology, and entanglements are inherited from the Hamiltonian. We apply this framework to explore the unconventional pairing symmetry and the Bogoliubov-de Gennes (BdG) equation in phase space. Our findings demonstrate that while superconductivity exhibits global coherence, the local Wannier orbital symmetry primarily determines the pairing symmetry. We analytically solve the spin-fluctuation-mediated pairing symmetry on the phase space by engineering a flat band with artificial gauge fields. We validate the model on the square-octagon superconductor ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$ using density functional theory (DFT), revealing the coexistence of nodeless $s^{\pm }$ and nodal $s_{z^2}$ pairing symmetries. This phase-space framework provides a robust, obstruction-free lattice model for complex many-body systems and their exotic excitations.

We investigate the superfluid phase transition in an $\mathrm{SU}\left(N\right)$-symmetric Fermi gas with $N$ distinct spin states using the functional renormalization group. To capture pairing phenomena beyond mean-field theory, we introduce an auxiliary bosonic field and employ the leading order of the derivative expansion of the partially bosonized effective average action. By discretizing the effective potential on a grid and numerically integrating the flow equations, we resolve the thermodynamic behavior near the transition. Our results reveal a fluctuation-induced first-order phase transition for $N\ge 4$, which is absent at the mean-field level. In the unitary regime, we provide quantitative predictions for the critical temperature as well as for the discontinuities in the superfluid gap and entropy density as functions of $N$. With increasing $N$, the critical temperature decreases, while the discontinuities become more pronounced, indicating a stronger first-order transition.

Trapped vortices in superconductors introduce residual resistance in superconducting radio-frequency (SRF) cavities and disrupt the operation of superconducting quantum and digital electronic circuits. Understanding the detailed dynamics of trapped vortices under oscillating magnetic fields is essential for advancing these technologies. We have developed a near-field magnetic microwave microscope to study the dynamics of a limited number of trapped vortices under the probe when stimulated by a localized rf magnetic field. By measuring the local second-harmonic response ($P_{2\mathrm{f}}$) at subfemto-Watt levels, we isolate signals exclusively arising from trapped vortices, excluding contributions from surface defects and Meissner screening currents. Toy models of niobium superconductor hosting vortex pinning sites are introduced and studied with time-dependent Ginzburg-Landau (TDGL) simulations of probe/sample interaction to better understand the measured second-harmonic response. The simulation results demonstrate that the second-harmonic response of trapped vortex motion under a localized rf magnetic field shares key features with the experimental data. This measurement technique provides access to vortex dynamics at the micrometer scale, such as depinning events and spatially resolved pinning properties, as demonstrated in measurements on a niobium film with an antidot flux pinning array.

Solid-state refrigeration promises for environmentally friendly cooling with high energy efficiency and compact design scalability. However, its refrigeration performance (e.g., isothermal entropy change and adiabatic temperature change) is inferior particularly near room temperature (RT), which greatly hinders the practical application. Utilizing first-principles calculations and a deep potential machine learning method, along with our proposed potential energy difference approach, we discover that $\mathrm{MPSnB}{\mathrm{r}}_3$, one type of organic-inorganic hybrid perovskite materials (OIHEMs), exhibits large reversible barocaloric effect (BCE) under RT conditions, with the isothermal entropy change ($|\mathrm{\Delta }{S}_{\mathrm{rev}}|$) and adiabatic temperature change ($|\mathrm{\Delta }{T}_{\mathrm{rev}}|$) reaching 105 $\mathrm{J}{\mathrm{K}}^{-1}\mathrm{k}{\mathrm{g}}^{-1}$ and 43 K, respectively, along with a high coefficient of performance (COP) up to 4, which surpass the overall performance of most barocaloric materials. We unveil a ferroelectric phase transition mechanism through an analysis of the evolution of the dynamic process between the methylphosphonium (MP) organic molecules and the $\mathrm{SnB}{\mathrm{r}}_3$ inorganic framework under different temperature and pressure. The giant BCE originates from a significant potential energy change across the pressure-induced phase transition, during which the vibrational entropy change ($S_{\mathrm{vib}}$) of the MP organic molecules contributes dominantly to the total entropy change. Moreover, pressure release facilitates the transfer of phonons from high frequency to low frequency, thereby enhancing the entropy of the system. The present work not only predicts superior barocaloric performance in OIHEMs but also provides important insights into the mechanism of tunable ferroelectric phase transition related to the BCE, which offers theoretical guidance for the design of novel materials for solid-state thermal management.

Specific heat studies of the ${\mathrm{Sr}}_{1-u}{\mathrm{Ba}}_u{\mathrm{Mn}}_{1-y}{\mathrm{Ti}}_y{\mathrm{O}}_3$ system polycrystalline samples performed by the relaxation and DSC methods over the temperature range 2–450 K are reported. Anomalies accompanying the antiferromagnetic-paramagnetic and ferroelectric-paraelectric phase transitions were measured. The system studied is a promising multiferroic material of the first type, in which a strong coupling between the magnetic and electric systems, related probably to the fact that the same ${\mathrm{Mn}}^{4+}$ ions are responsible for the two orderings, appears. Analysis of the specific heat anomaly at the magnetic transition was done using the advanced theory of the continuous transitions, in which the presence of higher order terms in the free energy is considered, and the parameters of the critical behavior of the system were estimated. The shape of this anomaly suggests that arguably, despite earlier predictions, the transition loses the continuous character and becomes the first-order process for the Ti-containing samples. For majority of the compositions, the anomaly accompanying the first-order high-temperature ferroelectric transition was found to be smeared and not well pronounced, which was tentatively ascribed to a wide temperature range of coexistence of the para- and ferroelectric phases and a small difference between entropies of both phases.

Iron-based van der Waals (vdW) ferromagnets with relatively high ordering temperatures are a current research focus due to their significance in fundamental physics and potential applications in spintronics. Competing magnetic interactions and anisotropies can give rise to nontrivial spin textures in these materials, resulting in novel topological features. ${\mathrm{Fe}}_4{\mathrm{GeTe}}_2$ (F4GT) is a nearly room-temperature vdW ferromagnet, well known for hosting a spin-reorientation transition (SRT) arising from the competition between perpendicular magnetic anisotropy (PMA) and shape anisotropy. In this work, we investigate the angle-dependent magneto-transport properties of F4GT single crystal nanoflakes. We report a large topological Hall effect (THE) associated with noncoplanar spin textures, persisting over a wide temperature range around SRT in presence of in-plane external magnetic field. Moreover, we find a thickness-sensitive THE signal for the $c$-axis orientation of the magnetic field at a low-temperature regime which is associated with a reentrant Lifshitz transition, suggesting interplay between magnetism and the Fermi surface in F4GT.

Within the rapidly expanding family of two-dimensional (2D) materials, 2D ferromagnetic (FM) Janus semiconductors are attracting increasing attention because of their particular potential as emergent multifunctional devices. However, the low Curie temperatures of the experimentally available semiconductors hinder progress. Robust FM superexchange interactions are a prerequisite to achieve high Curie temperatures. Experimentally, 2D FM semiconductors with Janus structures are usually synthesized from non-Janus structures through the so-called Janus engineering strategy. Along synthetic routes of Janus engineering, there is a complex interplay between the two distinct kinds of effects, i.e., lattice size and ligand electronegativity. To unravel the effects of Janus engineering, we computationally study the superexchange interactions in $2H\text{−}\mathrm{Fe}XY$ ($X,Y$= Cl, Br, I) semiconductors. By quantitatively separating the two distinct kinds of effects using strength indicators for superexchange interactions, we find that decreasing lattice size and ligand atoms with smaller electronegativity values can strengthen FM superexchange interactions. The summarized guidelines for enhancing Curie temperatures are useful for the rational design of 2D FM Janus semiconductors for high Curie temperatures by strengthening FM superexchange interactions.

Through first-principles calculations, we investigate the quantum-well resonance enhanced tunneling magnetoresistance (TMR) effect in magnetic tunnel junctions (MTJs) with interface oxidation, specifically examining Al/Fe/FeO/MgO/Fe and Ag/Al/Fe/FeO/MgO/Fe MTJs. Our study reveals that the Fe layer situated between Al and FeO exhibits spin-polarized quantum-well resonance states, arising from the combination between the partial confinement of ${\mathrm{\Delta }}_1$ electron at the Al/Fe interface and the full confinement at the FeO/MgO boundary. By changing the quantum-well thickness (controlled by Fe layer thickness) or interfacial phase shift (modulated via Al layer thickness), the quantum-well level of one spin-polarized state can be tuned near the Fermi level, leading to a high conductance in parallel state. This mechanism greatly enhances the optimistic TMR ratio from 400% in Fe/FeO/MgO/Fe MTJ to 3500% in Al/Fe/FeO/MgO/Fe MTJ and 9600% in Ag/Al/Fe/FeO/MgO/Fe MTJ. This study demonstrates that quantum-well resonance remains functionally robust under the interface oxidation—an advancement for practical spintronics applications.

The altermagnet exhibits the nonrelativistic spin splitting that enables all-electrical generation of spin-polarized currents beyond the spin-orbit coupling. Here, we report on a study on the anisotropic spin-polarized conductivity in collinear altermagnets. Based on the Boltzmann transport theory, we first study this effect using the general group-theoretical analysis and identify the spin point groups sustaining the finite spin polarization given in terms of spin-polarized conductivity. We show that the spin polarization vanishes along any direction for the $g$-wave and $i$-wave altermagnets while the spin polarization is significantly anisotropic for the $d$-wave altermagnet. We further derive the analytical expressions for the anisotropic spin polarization in the $d$-wave altermagnets. Then, we exemplify those phenomena in several representative altermagnets based on the density functional theory calculations. Our work enriches the altermagnetic spintronics and paves the practical way to produce large spin polarization in collinear altermagnets.

The magnetoresistance of superconductor–topological insulator–superconductor structures, with indium as the superconductor and ${\mathrm{TaSe}}_3$ as the topological insulator, shows steplike features on the resistance under magnetic fields. These resistance steps result from the suppression of superconductivity, induced by the superconducting proximity effect in both the bulk and surface states of the topological insulator. The position and amplitude of the steps, occurring at approximately 0.1 T, show an unusual dependence on the magnitude of the uniaxial strain ($\epsilon$), indicating their connection with surface states. This behavior follows the expected transition sequence, semimetal$\to$strong topological insulator$\to$trivial insulator, and supports the presence of surface states at $0.46\%\lesssim \epsilon \lesssim 0.85\%$.

We theoretically propose a possibility of realizing high-$T_c$ superconductivity having $s\pm$-wave symmetry in the heavily hole-doped infinite layer nickelates ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$. We consider situations where the original $P4/mmm$ symmetry of ${\mathrm{LaNiO}}_2$ is maintained even for a significant amount of Sr substitution by growing thin films on substrates having tetragonal symmetry. Considering such cases is indeed justified by our phonon calculations. For electron configurations somewhat close to $d^8$, the interaction between the $d_{x^2-y^2}$ band and the other $3d$ bands that lie just below the Fermi level results in an enhancement of superconductivity where the sign of the gap function is reversed between the former and the latter bands.

Rydberg atom arrays have emerged as a powerful platform for experimental research and a challenging subject for theoretical investigation in quantum science. In this study, we investigate the finite-temperature properties of two-dimensional square-lattice Rydberg atom arrays using the projected entangled pair states (PEPS) method. By analyzing the thermal behavior of systems in the checkerboard and striated phases, we extract critical exponents and identify phase transition characteristics. Our results confirm that the checkerboard phase transition belongs to the 2D Ising universality class, while the striated phase exhibits critical exponents that deviate from known universality classes, possibly due to finite-size effects. These findings provide theoretical insights into the thermal stability of quantum phases in Rydberg atom arrays and offer valuable guidance for future experimental efforts.

The momentum-resolved electronic structure of $\mathrm{Si} \left(111\right)\sqrt{3}\times \sqrt{3}\text{−}\mathrm{Sn}$, a surface Mott insulating system, was verified by in situ angle-resolved photoemission and spectroscopic-imaging scanning tunneling microscopy measurements. All the samples grown on different Si substrates were shown to be metallic at room temperature. However, discrepancies with the density functional theory calculation that do not consider correlation effects were observed, which suggests that there is some effect of correlation even though the Mott transition was reported to occur at 200 K. At 7 K the samples became insulating, but we found that the electron filling of this system can be controlled by repeated annealing of the Si substrate (flashing); the sample grown on a highly $p$-doped substrate became metallic due to hole doping whereas the the sample grown on a highly $n$-doped substrate became more insulating by increasing the number of flashing. The quasiparticle band dispersion with a small bandwidth near the Fermi level was clarified after hole doping and reproduced with tight-binding calculations.

This work investigates the Fermi surface of the Ti-doped ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ superconductor under pressures up to 2.1 GPa. Upon the application of both chemical and physical pressure in ${\mathrm{CsTi}}_{0.1}{\mathrm{V}}_{2.9}{\mathrm{Sb}}_5$, the superconducting transition temperature increases to $T_c=6.5$ K, nearly three times higher than that of the parent compound. To probe the Fermi surface under pressure, in situ magnetoresistance (MR) measurements are performed in magnetic fields up to 18 T at four pressure points: 0.25, 0.47, 1.55, and 2.1 GPa. The MR data at 0.25 GPa reveal clear Shubnikov–de Haas oscillations with frequency components at $F_{\alpha }=21$ T, $F_{\beta }=83$ T, $F_{\gamma }=709$ T, and $F_{\delta }=804$ T. These quantum oscillation frequencies, which correspond to extremal cross-sectional areas of the Fermi surface, vanish above 0.47 GPa, strongly indicating a pressure-induced Fermi surface reconstruction. To interpret these results, we conduct density functional theory calculations of the electronic band structure and Fermi surface. The calculations show a shift in Dirac points and Van Hove singularities near the Fermi level, along with notable changes in the Fermi surface topology as Ti doping increases, confirming that Ti substitution effectively tunes the electronic structure of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. Our combined experimental and theoretical study offers valuable insight into how both chemical and physical pressure can be used to engineer the electronic structure of ${\mathrm{CsTi}}_{0.1}{\mathrm{V}}_{2.9}{\mathrm{Sb}}_5$, providing a pathway for tuning correlated phenomena in kagome superconductors.

$\text{U}\left(1\right)$ gauge theories, including conventional Maxwell electromagnetism, allow $\theta$ terms when parity and time-reversal symmetries are broken. In condensed matter systems, the physics of $\theta$ as a magnetoelectric response has been explored extensively within the context of topological insulators and multiferroics. We show how $\theta$ terms can arise in the internal dynamics of the emergent electromagnetism in a $\text{U}\left(1\right)$ quantum spin liquid. In its Coulomb phase, the minimal model of pyrochlore quantum spin ice is governed by a six-spin ring exchange Hamiltonian. We identify the next-order contribution to the microscopic Hamiltonian when parity, time-reversal, and all improper spatial symmetries are broken, a seven-spin term which leads to a two-parameter lattice gauge theory with a $\theta$-electromagnetic phase. We derive how the seven-spin term is generated perturbatively within each of the three symmetry classes of short-range pyrochlore spin ice. Within a complete microscopic symmetry analysis, we find that the most general nearest-neighbor Hamiltonians fail to generate the seven-spin term, and one must include next-nearest-neighbor interactions to obtain an emergent $\theta$. Using gauge mean-field theory, we compute additional contributions to the $\theta$ term from the spinon sector. Finally, we determine the conditions required for an internal $\theta$ term to generate a significant external magnetoelectric response.

Screened spherical wave (SSW) of the Hankel function features the complete, minimal, and short-ranged basis set, presenting a compact representation for electronic systems. In this work, we report the implementation of full-potential (FP) SSW-based tight-binding linearized muffin-tin orbital (TB-LMTO) for all-electron density functional theory (DFT), and provide extensive tests on the robustness of FP-TB-LMTO and its high accuracy for first-principles material simulation. Through the introduction of double augmentation, SSW-based MTO is accurately represented on the double grids including the full-space uniform and dense radial grids. Based on the double augmentation, the accurate computation of full charge density, full potential, complex integral in the interstitial region, and the total energy are all effectively addressed to realize the FP-TB-LMTO for DFT self-consistent calculations. By calculating the total energy, band structure, phase ordering, and elastic constants for a wide variety of materials, including normal metals, compounds, and diamond-structured silicon, we demonstrate the highly accurate numerical implementation of FP-TB-LMTO for all-electron DFT in comparison with other well-established FP method. The implementation of FP-TB-LMTO based DFT offers an important tool for the accurate first-principles tight-binding electronic structure calculations, particularly important for the large-scale or strongly correlated materials.

We analyze mass renormalization in massive Dirac-like systems in ($2+1$) dimensions arising from electron-phonon interactions at finite temperatures, employing the large-$N$ expansion. Our model combines the low-energy description of charge carriers in a buckled honeycomb lattice with the low-energy approximation for phonons and electron-phonon interactions in two-dimensional materials. Hence the system is modeled as a massive Dirac-like field coupled to a two-component vector field ${\mathcal{A}}_i$, representing the phonon modes. This framework allows us to compute the one-loop electron self-energy at finite temperature, from which we derive the renormalized electronic band gap, $2m^R$. The effective model is subsequently applied to describe the renormalized optical band gap $\left(E_{\mathrm{opt}}\right)$ in monolayers of transition metal dichalcogenides (TMDs), including ${\mathrm{MoS}}_2$, ${\mathrm{MoSe}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$, using the relation $E_{\mathrm{opt}}=2m^R-\left|E_b\right|$, where $|E_b|$ is the exciton binding energy that remains constant at the examined temperature. A good agreement is observed with experimental data for reasonable values of the ultraviolet cutoff, $\mathrm{\Lambda }\approx 1$ eV. Our main findings indicate that $E_{\mathrm{opt}}$ remains nearly constant at low temperatures, whereas at higher temperatures it decreases linearly with the temperature $T$. Specifically, we find that $E_{\mathrm{opt}}$ reduces by approximately $\approx [0.1,0.2]$ eV as the temperature increases from $\approx 4$ K to 500 K, consistent with recent experimental observations. Furthermore, we estimate the temperature range at which the transition to the linear regime occurs, obtaining typical values within $\approx [110,150]$ K for the four materials under consideration.