We analyze the role of spatial electronic correlations and, in particular, of the magnetic fluctuations in Mott insulators. A half-filled Hubbard model is solved at large strength of the repulsion $U$ on a two-dimensional square lattice using an advanced diagrammatic approach capable of going beyond Hartree-Fock and single-site dynamical mean-field theories. We show that at high temperatures, when the magnetic fluctuations are weak, the electronic self-energy of the system is mainly local and is well reproduced by the atomic (Hubbard-I) approximation. Upon lowering the temperature toward the magnetically ordered phase, the nonlocality of the self-energy becomes crucial in determining the momentum dispersion of the Hubbard bands and the Green's function zeros. We therefore establish a precise link between Luttinger surface, nonlocal correlations and spectral properties of the Hubbard bands.

We successfully synthesized ${\left(p\text{−}\mathrm{Py}\text{−}\mathrm{V}\right)}_2\left[\mathrm{Cu}{\left(\mathrm{hfac}\right)}_2\right]$, a verdazyl-based complex. Molecular orbital calculations revealed that three antiferromagnetic (AF) exchange interactions form a spin-$\frac{1}{2}$ lattice with coordination numbers 2 and 4. The magnetic susceptibility and specific heat exhibit contributions of AF correlations and a phase transition to an AF ordered state at $T_{\mathrm{N}}=3.6$ K. The magnetization curve exhibits an almost linear increase in the low-field region for $T>T_{\mathrm{N}}$. Furthermore, we observe a nonlinear increase in the high-field region, demonstrating the magnetic moment reduction caused by the strong quantum fluctuations. Through numerical analysis, we reveal a hump structure of magnetic susceptibility and a 1/3 magnetization plateau that emerge owing to the effective spin state. These results provide a model compound forming a spin-$\frac{1}{2}$ mixed coordination lattice and an understanding of its unique quantum properties attributed to strong quantum fluctuations.

FeSi is a curious example of a $d$-electron system that manifests many of the same phenomena associated with $f$-electron Kondo insulators, including conducting surface states with potentially nontrivial topology. Here we investigate the magnetization and magnetotransport of these surface states and how a 2D ferromagnetic state at the surface of FeSi influences the surface conductivity. We confirm the 2D ferromagnetism via a systematic study of magnetization on groups of filtered fragments with increasing surface area-to-volume ratios, identifying characteristic temperatures and magnetic fields associated with the ordered state. The paramagnetic to ferromagnetic transition appears broadened, suggesting disorder, which allows spin fluctuations to manifest up to at least 9 T at 2 K. This highlights the need to understand the relation between the disorder of the 2D ferromagnetism and the surface conductivity in FeSi.

The propagation of magnons along a symmetry path may depend on the direction of propagation, similar to many other quasiparticles in nature. This phenomenon is commonly referred to as nonreciprocity. In addition to the fact that it is of great interest to understand the fundamental physical mechanism leading to this nonreciprocal propagation, the phenomenon of magnon nonreciprocity may be used to design magnon-based logic devices. Recently, it has been demonstrated that a significantly large spin-orbit coupling can lead to giant nonreciprocity of exchange-dominated terahertz magnons, when they are excited by means of spin-polarized electrons [Phys. Rev. Lett. 132, 126702 (2024)]. Here, by providing experimental results on two additional systems we demonstrate the generality of the observed phenomenon. Comparing the results of a Co/Ni bilayer on Ir(001) to those of a Co double layer on Ir(001) and W(110), we unravel the impact of the interfacial electronic hybridization on the observed phenomenon and provide further insights into the microscopic mechanism leading to this nonreciprocal magnon excitation. It was observed that the interfacial electronic hybridization is of some importance but is not crucial for the magnon nonreciprocity. This is an important observation since the electronic hybridization is known to be a key aspect in the determination of the magnetic properties at the interface. On the other hand, the choice of the incident energy of the incoming electron beam is decisive for the observation of the effect. Our results indicate that depending on the energy of the incident electron beam and the scattering geometry the magnon nonreciprocity can be tuned and even be inverted for some ranges of the magnon momentum. The results may inspire new ideas for the generation of nonreciprocal ultrafast terahertz magnons in ultrathin films of $3d$ magnetic metals as well as in two-dimensional van der Waals magnets, interfaced with solids possessing a large spin-orbit coupling, for applications in the field of magnonics.

We investigate the properties of a Fibonacci quasicrystal (QC) arrangement of a one-dimensional topological superconductor, such as a magnetic atom chain deposited on a superconducting surface. We uncover a general mutually exclusive competition between the QC properties and the topological superconducting phase with Majorana bound states (MBS): there are no MBS inside the QC gaps and the MBS never behave as QC subgap states and, likewise, no critical or winding QC subgap states exist inside the topological superconducting gaps. Surprisingly, despite this competition, we find that the QC is still highly beneficial for realizing topological superconductivity with MBS. It both leads to additional large nontrivial regions with MBS in parameter space, that are topologically trivial in crystalline systems, and increases the topological gap protecting the MBS. We also find that shorter approximants of the Fibonacci QC display the largest benefits. As a consequence, our results promote QCs, and especially their short approximants, as an appealing platform for improved experimental possibilities to realize MBS as well as generally highlight the fundamental interplay between different topologies.

The observance of vortex and antivortex effects in bulk crystals can prove the existence of phase-disordered superconductivity in the bulk. To gain insights into the mechanisms that govern superconducting transition in copper oxide high-transition temperature $\left(T_c\right)$ superconductors, this study investigated the transport properties of underdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{10+\delta }$ (Bi-2223) bulk single crystals. The $I\text{−}V$ characteristics results and the typical tailing behavior owing to the temperature dependence of in-plane resistivity $\left({\rho }_{ab}\right)$ were consistent with the Kosterlitz-Thouless (KT) transition characteristics. Thus, with increasing temperature, copper oxide high-$T_c$ superconductors transitioned to their normal state owing to destruction of their phase correlations, although a finite Cooper pair density was prevalent at $T_c$. Further, magnetization measurements were performed to determine the temperature dependence of the irreversible magnetic field $B_{irr}$. Consequently, the mechanism governing the KT transition-like superconducting transition in this bulk system was elucidated. These results support the extreme strong-coupling models for high-$T_c$ superconductivity in cuprates.

Square-triangle-rhombus ($\mathcal{STR}$) tilings are encountered in various self-organized multicomponent systems. They exhibit a rich structural diversity, encompassing both periodic tilings and long-range ordered quasicrystals, depending on the proportions of the three tiles and their orientation distributions. We derive a general scheme for characterizing $\mathcal{STR}$ tilings based on their lift into a four-dimensional hyperspace. In this approach, the average hyperslope ($2\times 2$) matrix $\mathcal{H}$ of a patch defines its global composition with four real coefficients: $\mathcal{X}, \mathcal{Y}, \mathcal{Z}$, and $\mathcal{W}$. The matrix $\mathcal{H}$ can be computed either directly from the area-weighted average of the hyperslopes of individual tiles or indirectly from the border of the patch alone. The coefficient $\mathcal{W}$ plays a special role as it depends solely on the rhombus tiles and encapsulates a topological charge, which remains invariant upon local reconstructions in the tiling. For instance, a square can transform into a pair of rhombuses with opposite topological charges, giving rise to local modes with five degrees of freedom. We exemplify this method for $\mathcal{STR}$ tilings through its application to experimental structures observed in two-dimensional Ba-Ti-O films on metal substrates, demonstrating the hyperslope matrix $\mathcal{H}$ as a precise tool for structural analysis and characterization.

The control of heat flow in disordered materials presents a significant challenge due to the limitations of conventional phonon transport models in systems lacking periodic long-range crystal order. This study investigates energy dissipation mechanisms induced by structural irregularities, utilizing folded polymers, particularly proteins, as model systems. Proteins, macromolecules characterized by coexisting periodic amino acid chains folded into irregular three-dimensional structures, serve as useful platforms for examining the impact of irregular topologies on vibrational properties. Our research reveals an important enhancement of the phonon density of states at mid-band frequencies, diverging from the van Hove singularities typically expected at Brillouin-zone edges in perfect crystals. This state redistribution exhibits similarities to observations in some disordered electronic and optical systems, generally known as Lifshitz tails. By interpreting this effect as a resonance between multiple degrees of freedom tuned by gradients of an effective phonon confinement potential, we provide a rational for interpreting the ubiquitous “boson peak” reported in disordered materials. Furthermore, this study elucidates how disorder allows heat to be channeled in narrow frequency bands. To this purposes, we present mathematical tools that enable rapid and sharp estimation of the phonon density of states and thermal currents, circumventing the need for solving expensive eigenvalue problems. Our methodology may facilitate the characterization and control of heat transport in specific amorphous and disordered solids, with implications for tailoring thermal materials through strategic manipulation of structural disorder.

With exceptionally low magnetic damping, YIG has been extensively applied in the realm of magnetism, encompassing the researches into the spin Seebeck effect. YIG has 20 magnon bands, including 8 higher-energy bands denoted ${\alpha }_1-{\alpha }_8$, and 12 lower-energy bands denoted ${\beta }_1-{\beta }_{12}$. Here, we study the impact of the complex intraband and interband magnon couplings on the transport coefficients of YIG. Four-magnon processes in YIG are considered, and a self-consistent mean-field approximation is made for these interaction terms. We find that the $\beta$ bands exhibit minimal variation with increasing temperature, whereas the $\alpha$ bands undergo a noticeable decline as the temperature rises. These counterintuitive results agree well with the observation of earlier inelastic neutron-scattering experiments and the results of the theoretical calculations in recent years. We also find it sufficient to include only the contribution of magnons on the acoustic band ${\beta }_1$ when studying the spin conductivity ${\sigma }_m$. However, when calculating the spin Seebeck coefficient $S_m$ and the magnon thermal conductivity ${\kappa }_m$, the results calculated using only ${\beta }_1$ show noticeable deviations over a large temperature range compared with the full band calculations. These deviations are well mitigated when the ${\beta }_2$ and ${\beta }_3$ bands are considered. Finally, we find our results satisfy the Wiedemann-Franz law for magnon transport.

The evolution of the critical superconducting temperature and field, quantum oscillation frequencies, and effective mass $m^{*}$ in underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) crystals ($p=0.11$, with $p$ the hole concentration per Cu atom) points to a partial suppression of the charge orders with increasing pressure up to 7 GPa, mimicking doping. Application of pressures up to 25 GPa pushes the sample to the overdoped side of the superconducting dome. In contrast to other cuprates, or to doping studies on YBCO, the frequencies of the quantum oscillations measured in that pressure range do not support the picture of a Fermi-surface reconstruction in the overdoped regime, but possibly point to the existence of a new charge order.

Strange metal behavior is traditionally associated with an underlying putative quantum critical point at zero temperature. However, in many correlated metals, e.g., high-$T_c$ cuprate superconductors, strange metallicity persists at low temperatures over an extended range of microscopic parameters, suggesting the existence of an underlying quantum critical phase, whose possible physical origins remain poorly understood. Systematic investigations of physical scenarios giving rise to such a critical, non-Fermi liquid (NFL) phase are therefore crucial to better understand this puzzling behavior. In a previous study [Bashan et al., Phys. Rev. Lett. 132, 236501 (2024)], we considered a solvable large-$N$ model consisting of itinerant electrons coupled to local two-level systems (TLSs) via spatially random interactions, inspired by the possibility of emergent metallic glassiness due to frustrated competing orders, and found that the system hosts an NFL phase with tunable exponents at intermediate couplings. In this paper, we expand our investigation to the following: (i) We study the extent to which this NFL phase is generic by considering various deformations of our theory, including coupling of electrons to multiple operators of the TLSs and arbitrarily directed TLS fields. We find that the physical picture obtained in Bashan et al. [Phys. Rev. Lett. 132, 236501 (2024)] qualitatively persist in a wide region of parameter space, showcasing the robustness of the NFL phase. (ii) We analyze the superconducting instability caused by the coupling of TLSs to electrons, and find a rich structure, including quantum critical pairing associated with the NFL phase and conventional BCS-like pairing in the weak and strong coupling limits. (iii) We elaborate on the analysis of Bashan et al. [Phys. Rev. Lett. 132, 236501 (2024)], including single-particle, transport, and thermodynamic properties.

Electron-electron collisions lead to a T-square component in the electrical resistivity of Fermi liquids. The case of liquid ${}^3\mathrm{He}$ illustrates that the thermal resistivity of a Fermi liquid has a T-square term, expressed in $\mathrm{m}\times {\mathrm{W}}^{-1}$. Its natural units are $\hslash /k_F{E}_F^2$. Here, we present a high-resolution study of the thermal conductivity in bismuth, employing magnetic field to extract the tiny electronic component of the total thermal conductivity and resolving signals as small as $\approx 60\mu \mathrm{K}$. We find that the electronic thermal resistivity follows a T-square temperature dependence with a prefactor twice larger than the electric T-square prefactor. Adding this information to what has been known for other semimetals, we find that the prefactor of the T-square thermal resistivity scales with the square of the inverse of the Fermi temperature, implying that the dimensionless fermion-fermion collision cross section is roughly proportional to the Fermi wavelength, indicating that it is not simply set by the strength of the Coulomb interaction.

The absence of screening of the nonlocal Coulomb interaction in Dirac systems at charge neutrality leads to the breakdown of the Fermi liquid and divergence of the Fermi velocity. On the other hand, Mott-Hubbard physics and the concomitant formation of local moments is dominated by the local effective Hubbard interaction. Using quantum Monte Carlo methods combined with stochastic analytical continuation, we compute the single particle spectral function of fermions on the honeycomb lattice for a realistic interaction that includes both the Hubbard interaction and long-ranged Coulomb repulsion. To a first approximation, we find that the generic high-energy features, such as the formation of the upper Hubbard band near the phase transition, are primarily determined by the local effective Hubbard interaction. In the weakly interacting regime, the long-range Coulomb interaction enhances the bandwidth of quasiparticles and suppresses their lifetime. Conversely, near the phase transition, the long-range Coulomb interaction suppresses the background antiferromagnetic fluctuation, which potentially promotes the propagation of spin polarons, leading to a slight enhancement of the quasiparticle spectral weight and lifetime.

A Wigner chain in a periodic potential is a paradigmatic example of geometric frustration with long-range interactions. The dynamics emulates the Frenkel-Kontorova model with Coulomb interactions. In the continuum approximation, dislocations are sine-Gordon solitons with power-law decaying tails. We show that their action is mapped into a massive, long-range ($1+1$) Thirring model, in which the solitons are charged fermionic excitations over an effective Dirac sea. We identify the corresponding mean-field theory and show that the Coulomb interactions destabilize structures commensurate with the periodic substrate, suppressing their onset and giving rise to interaction-induced lubrication. Our study identifies the role of long-range interactions on determining nanofriction. Our predictions can be probed in state-of-the-art trapped ion experiments.