We employ spectroscopic-imaging scanning tunneling microscopy to investigate the electronic properties of copper-oxide (${\mathrm{CuO}}_2$) planes in an electron-doped infinite-layer cuprate superconductor ${\mathrm{Sr}}_{1-x}{\mathrm{Nd}}_x{\mathrm{CuO}}_2$. Tunneling conductance spectra reveal distinct Bose-Einstein condensation (BEC)-like features in underdoped regions, characterized by a small Fermi energy, broad coherence peaks, and pronounced particle-hole asymmetry. As the local electron doping level increases, these features evolve into well-defined superconducting gaps, unveiling the long-sought crossover between the BEC and Bardeen-Cooper-Schrieffer (BCS) regions in cuprate superconductors. Additionally, we observe three distinct phonon excitations within the BEC regions, which underscores the potential role of lattice vibrations in electron pairing. Our results provide direct real-space experimental evidence for the BEC-BCS crossover on ${\mathrm{CuO}}_2$ planes, offering insights into the emergent superconductivity in doped Mott cuprates.

We study the electron addition spectrum of the one-dimensional extended Hubbard-Su-Schrieffer-Heeger (HSSH) model in the dilute limit using the density matrix renormalization group method. In addition to the expected renormalization to the band structure, we find that the electron-phonon ($e$-ph) interaction produces an anomalous spectral feature when electrons are added in the singlet channel but which is absent in the triplet channel. By comparing these results with those obtained from perturbation theory in the antiadiabatic limit, we demonstrate that this anomalous feature is a remnant of the strong electron-electron interaction mediated by the SSH coupling previously derived in the two-particle limit. By studying the evolution of this feature as a function of doping, we track the fate of this attraction to higher carrier concentrations and provide predictions for the spectral features to help guide future searches for strong $e$-ph mediated pairing.

Universal conductance fluctuations (UCF) are a hallmark of quantum interference in mesoscopic devices. According to the Altshuler-Lee-Stone theory, the amplitude of UCF remains independent of system parameters such as Fermi energy and disorder strength. However, recent experiments have demonstrated a significant variation in UCF with respect to Fermi energy in the anisotropic Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$, suggesting a dependence on band anisotropy. In this work, we reconcile the discrepancy between theoretical predictions and experimental observations through a detailed study of UCF versus Fermi energy using a tight-binding model with tunable anisotropy parameters. Near the band edge, the Hamiltonian is simplified to an anisotropic free electron gas model, recovering the generalized Altshuler-Lee-Stone theory. However, as the Fermi energy shifts toward the band center, where rotational symmetry breaks into $C_4$ (fourfold rotational) symmetry, the UCF amplitude deviates from the standard theory. Our findings reveal that UCF becomes increasingly sensitive to Fermi energy as the anisotropy grows stronger. Furthermore, using realistic parameters for ${\mathrm{Cd}}_3{\mathrm{As}}_2$, our calculations demonstrate an increase in UCF away from the Dirac point, in qualitative agreement with experimental results. The enhancement of UCF occurs in two perpendicular transport directions that we have calculated, albeit with quantitative differences in magnitude, which can be tested in future experiments. Given the prevalence of anisotropic materials and technical advances in engineering anisotropy through strain or twist, our results offer a valuable reference for characterizing intrinsic electronic properties via UCF.

While the dynamics of fully connected systems is dominated by mean-field effect, in the classical limit the single-particle observables are observed to relax toward a nonthermal stationary value. This phenomenon is known as violent relaxation and it is generally absent at the quantum-mechanical level, where single-particle observables exhibits long-lived oscillations. In this paper we explain this discrepancy by determining some very restrictive conditions that quantum single-site Hamiltonians should meet in order for the system to undergo relaxation. We thus check them by introducing a new model (the so-called $w$ model) which can exhibit dynamical phase transition among thermal behavior, persistent excitations, and violent relaxation. We also propose a way to implement it within light-matter coupling. We finally explain how the classical limit restore this behavior, thus showing that, even if the mean-field dynamics of quantum models is usually thought to be classical, quantum effects still play an important role in it.

Interaction of magnetic metasurfaces with common magnetic dielectrics is of high interest from both the fundamental and practical points of view. Control over the magnetic domain structure of magneto-optical materials is of principal importance for a number of applications. Interaction of garnets with overlying ferromagnetic structures can provide an additional avenue for the domain manipulation. In this study, we demonstrate that ferromagnetic Co/Au gratings on the surface of rare-earth-substituted iron garnet films have a strong influence on the orientation and width of the stripe domains (SDs), as revealed by optical microscopy and magneto-optical studies. Namely, we show (i) a tendency of the SDs alignment along the Co/Au stripes that are turned by $5^{\circ }–{20}^{\circ }$ from the direction of the external magnetic field and (ii) plateaulike dependencies of the SDs' width on the field strength, which are not specific for pristine garnet. The key mechanism here is the magnetostatic interaction of the ferromagnetic stripes and the garnet surface, which allowed us to tame the domain structure and to fix their in-plane position. These findings allow us to overcome the chaotic domains orientation and thus increase the stability of magneto-optical devices.

We have investigated the magnetic state of $\lambda$-(BEDT-STF)${}_2{\mathrm{GaCl}}_4$ [BEDT-STF = bis(ethylenedithio)diselenadithiafulvalene], which is located in the vicinity of the Mott boundary, using ${}^{69/71}\mathrm{Ga}$ NMR measurements. No divergent peak in the temperature dependence of the nuclear spin-lattice relaxation rate, 1/$T_1$, is observed down to 1.8 K, suggesting that no long-range magnetic ordering occurs even at low temperatures. Instead, the gradual development of the spin inhomogeneity toward low temperatures was confirmed by the temperature dependence of the linewidth, 1/$T_1$, and the stretched exponent $\beta$. We discussed the possible origin that causes the inhomogeneous state to emerge in the vicinity of the Mott boundary.

${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) has favorable macroscopic superconducting properties of $T_{\mathrm{c}}$ up to 93 K and $H_{c2}$ up to 150 T. However, its nanoscale electronic structure remains mysterious because bulk-like electronic properties are not preserved near the surface of cleaved samples for easy access by local or surface-sensitive probes. It has been hypothesized that Ca-doping at the Y site could induce an alternate cleavage plane that mitigates this issue. We use scanning tunneling microscopy (STM) to study both Ca-free and 10% Ca-doped YBCO . We provide experimental evidence, supported by density functional theory (DFT) calculations, that the Ca-doped samples do indeed cleave on an alternate plane, yielding a spatially disordered partial (Y, Ca) surface. On this surface, we image a superconducting gap with an average value of 26 meV $\pm$ 4 meV and characteristic length scale of around 1 nm, similar to Bi-based high-$T_{\mathrm{c}}$ cuprates, and the first map of gap inhomogeneity in YBCO.

Sliding ferroelectricity has garnered significant research interest in recent years, and the foundational bilayer stacking ferroelectricity (BSF) theory [Ji et al., Phys. Rev. Lett. 130, 146801 (2023)] provides valuable guidelines for the design of sliding ferroelectrics. However, the current framework does not fully account for the selection of rotation operators in bilayer stacking, and is unable to determine whether and how the direction of polarization can be switched in polar bilayers. Here, we introduce two key improvements to the BSF theory through a systematic group theory analysis. First, we develop a more general method for identifying possible rotation operators, enabling a refined classification of the symmetries and polar states in bilayers that arise when stacking monolayers from oblique and rectangular crystal systems. Second, we establish an approach to determine the symmetry operators that connect the energetically degenerate configurations, thereby integrating the determination of ferroelectric switching into the BSF theory. These enhancements make the theory more comprehensive, providing a more robust and practical foundation for the design of sliding ferroelectrics.

The celebrated Kitaev chain reveals a captivating phase diagram in the presence of various disorders, encompassing multifractal states and topological Anderson phases. In this paper, we investigate the localization and topological properties of a dimerized topological noncentrosymmetric superconductor (NCS) under quasiperiodic and Anderson disorders. Using both global and local characterization methods, we identify energy-dependent transitions from ergodic to multifractal and localized states. Extended multifractal regimes emerge from the competition between dimerization, NCS order, and quasiperiodic modulation. This interplay causes localization to occur preferentially in different energy bands depending on the disorder strength, with the lowest bands exhibiting the highest sensitivity to parameter variations. We employ the real-space polarization method to compute the ${\mathbb{Z}}_2$ topological invariant, revealing alternating topological and trivial phases as the quasiperiodic potential increases, a behavior distinct from the typical topological Anderson phase diagram. Additionally, the topological states show remarkable robustness against Anderson disorder, providing insights into topological phase stability in noncentrosymmetric systems. Finally, we propose a feasible experimental scheme based on superconducting Josephson junctions, where NCS-like behavior can be engineered via spatially modulated supercurrents. Our findings highlight the distinct roles of different disorder types in shaping localization and topology, providing insight into the engineering of Majorana zero modes and offering profound implications for topological quantum encryption schemes.

We report the magnetoelastic properties of a ${\mathrm{Ba}}_2{\mathrm{CuGe}}_2{\mathrm{O}}_7$ single crystal at low temperatures under a magnetic field applied along the crystallographic [001] axis. Our results extend to low temperatures, the $H-T$ phase diagram determined for this compound by neutron scattering. Furthermore, we observe that specific elastic modes are more sensitive to the various magnetic transitions. In particular, we observe an unusual coupling between the transverse acoustic mode in the (001) plane and the cycloidal order at low field, which suggests a spin-strain mechanism originating from Dzyaloshinskii-Moriya interaction in this compound.

Building on the principle of combinatorial gauge symmetry, lattice gauge theories can be formulated with only one- and two-body interactions that ensure the exact realization of the symmetry rather than its approximate emergence in a perturbative regime. This paper extends the framework to encompass generic non-Abelian finite gauge groups by expanding on previous work that developed the theory for finite Abelian gauge groups and presented one non-Abelian example.

Projected entangled-pair states (PEPS) have become a powerful tool for studying quantum many-body systems in the condensed matter and quantum materials context, particularly with advances in variational energy optimization methods. A key challenge within this framework is the computational cost associated with the contraction of the two-dimensional lattice, crucial for calculating state vector norms and expectation values. The conventional approach, using the corner transfer matrix renormalization group (CTMRG), involves combining two tensor network layers, resulting in significant time and memory demands. In this work, we introduce an alternative split-CTMRG algorithm, which maintains separate PEPS layers and leverages modified environment tensors, reducing computational complexity while preserving accuracy. Benchmarks on quantum lattice models demonstrate substantial speedups for variational energy optimization, rendering this method valuable for large-scale PEPS simulations.

Driven by the growing interest in double-half-Heusler structures known for their lower thermal conductivity compared to traditional half-Heusler materials, this study employs first-principles computational calculations to explore the thermoelectric properties of $X_2{\mathrm{FeNiSn}}_2$ ($X=\mathrm{Ta},\mathrm{Nb} \text{and} \mathrm{V}$) compounds. We analyzed the structural, mechanical, electronic, and vibrational properties of these structures and calculated their thermoelectric properties using the Boltzmann transport theory for electron and phonon transport. The electronic transport properties are also comparatively studied with constant relaxation time approximation, full-band relaxation time approximation, and energy-dependent phonon- and impurity-limited carrier scattering time approximation. Our findings indicate that considering electron scattering from all phonon branches is critical for accurately assessing the transport properties of these materials, resulting in significantly different $ZT$ values compared to those obtained using the constant and full-band relaxation time approximations.

While the exact phase diagram of the Fermi-Hubbard model remains poorly understood despite decades of progress, nearly 60 years ago Nagaoka proved that a single dopant in an otherwise half-filled Hubbard system can bring about ferromagnetism through kinetic means. The phenomenon was recently observed with ultracold atoms in triangular optical lattices. Here, we explore the kinetic ferromagnetism within the square lattice Hubbard model and its strong-coupling counterpart, the $t-J$ model, at finite temperatures in the thermodynamic limit via numerical linked-cluster expansions. We find evidence of ferromagnetic Nagaoka polarons at dopings up to $\sim 30\%$ away from half filling for a variety of interaction strengths and at temperatures as low as 0.2 of the hopping energy. We map out the boundaries of this phase through analyzing various correlation functions.

This work focuses on the layered perovskite ${\mathrm{Sr}}_2{\mathrm{NbO}}_4$, a $4d$ analog of ${\mathrm{Sr}}_2{\mathrm{VO}}_4$, which remains an unsolved puzzle with a possible intriguing hidden magnetic order. Using density functional theory (DFT) calculations, we demonstrate the robust thermodynamic stability and exfoliability of ${\mathrm{Sr}}_2{\mathrm{NbO}}_4$, suggesting potential applications as a 2D material. Imperfect Fermi surface nesting indicates instabilities that may drive symmetry lowering, charge/orbital density waves, or superconductivity. Dynamical mean-field theory (DMFT) calculations reveal moderate mass renormalization $(m^{*}/m\sim 1.3)$ and an itinerant character of magnetism with strong longitudinal spin fluctuations. The exchange interaction is dominated by in-plane ferromagnetic coupling with much weaker interlayer antiferromagnetic exchange.

We report on a tunable Josephson junction formed by a bilayer graphene ribbon encapsulated in ${\mathrm{WSe}}_2$ with superconducting niobium contacts. We characterize the junction by measurements of the magnetic field–induced interference pattern and the AC Josephson effect manifested as Shapiro steps, examining current-dependent hysteresis and junction dynamics. The latter can be tuned by temperature, gate voltage, and magnetic field. Finally, we examine the evolution of the supercurrent when subjected to in-plane magnetic fields. Notably, we observe strong anisotropy in the supercurrent with respect to the orientation of the in-plane magnetic field. When the field is parallel to the current direction, the supercurrent is suppressed and shows revivals with increasing magnetic field, whereas it remains almost unaffected when the field is oriented in a perpendicular direction. We suggest that this anisotropy is caused by the dependence of supercurrent interference on the junction geometry.

A quantitative understanding of the microscopic mechanisms responsible for damping in van der Waals nanomechanical resonators remains elusive. In this work, we investigate van der Waals magnets, where the thermal expansion coefficient exhibits an anomaly at the magnetic phase transition due to magnetoelastic coupling. Thermal expansion mediates the coupling between mechanical strain and heat flow and determines the strength of thermoelastic damping (TED). Consequently, variations in the thermal expansion coefficient are reflected directly in TED, motivating our focus on this mechanism. We extend existing TED models to incorporate anisotropic thermal conduction, a critical property of van der Waals materials. By combining the thermodynamic properties of the resonator material with the anisotropic TED model, we examine dissipation as a function of temperature. Our findings reveal a pronounced impact of the phase transition on dissipation, along with transitions between distinct dissipation regimes controlled by geometry and the relative contributions of in-plane and out-of-plane thermal conductivity. These regimes are characterized by the resonant interplay between strain and in-plane or through-plane heat propagation. To validate our theory, we compare it to experimental data of the temperature-dependent mechanical resonances of ${\mathrm{FePS}}_3$ resonators.