Recently discovered kagome superconductors $A{\text{V}}_3{\text{Sb}}_5$ $(A=\mathrm{K}, \mathrm{Rb}, \mathrm{Cs})$ exhibit exotic bulk and surface physical properties such as charge density wave (CDW) and chirality, whereas their origins remain unresolved. By using microfocused angle-resolved photoemission spectroscopy, we discovered that $A{\text{V}}_3{\text{Sb}}_5$ commonly exhibits two distinct polar surfaces depending on the termination: electron- and hole-doped ones for the $A$ and Sb termination, respectively. We observed that the kagome-derived band shows a splitting in the $A$-terminated surface, while it is absent in the Sb-terminated counterpart, suggestive of a signature of the polarity-dependent CDW at the surface. Close comparison of the band-dependent splitting implies that the three-dimensional CDW structure of the K-terminated surface is different from that of the Rb- or Cs-terminated surface, suggesting the possible diversity of the CDW ground state. These results provide important insight into the origin of CDW in kagome superconductors $A{\text{V}}_3{\text{Sb}}_5$.

Initial characterizations of the fermion sign problem focused on its evolution with spatial lattice size $L$ and inverse temperature $\beta$, emphasizing the implications of the exponential nature of the decay of the average sign $\langle \mathcal{S}\rangle$ for the complexity of its solution and associated limitations of quantum Monte Carlo studies of strongly correlated materials. Early interest was also on the evolution of $\langle \mathcal{S}\rangle$ with density $\rho$, either because commensurate filling is often associated with special symmetries for which the sign problem is absent, or because particular fillings are often primary targets, e.g., those densities, which maximize superconducting transition temperature (the top of the “dome” of cuprate systems). Here we describe an analysis of the sign problem, which demonstrates that the spin-resolved sign $\langle {\mathcal{S}}_{\sigma }\rangle$ already possesses signatures of universal behavior traditionally associated with order parameters, even in the absence of symmetry protection that makes $\langle \mathcal{S}\rangle =1$. When appropriately scaled, $\langle {\mathcal{S}}_{\sigma }\rangle$ exhibits universal crossings and data collapse. Moreover, we show these behaviors occur in the vicinity of quantum critical points of three well-understood models, exhibiting either second-order or Kosterlitz-Thouless phase transitions. Our results pave the way for using the average sign as a minimal correlator that can potentially describe quantum criticality in a variety of fermionic many-body problems.

The reduced dimensions of nanostructures offer opportunities to control and improve the performance of thermoelectric devices based on two-dimensional materials. Here, we investigate theoretically the thermoelectric properties of gated phosphorene junctions (GPJs). We focus on the electronic contribution due to the possible ultralow phononic thermal conductivity in GPJs. The effects of nanostructuration, temperature, and phosphorene anisotropy are analyzed. The hybrid matrix method and the Landauer-Büttiker formalism are implemented to obtain the transmission coefficient and thermoelectric properties, respectively. We consider electronic transport mediated by holes or equivalently $n-p-n$ transport, finding that nanostructuration induces a strong anisotropy in all thermoelectric properties. The Seebeck coefficient, figure of merit, and efficiency have a better response in the zigzag direction, whereas, the electronic conductance, power factor and electronic thermal conductance have higher values in the armchair direction. In the case of large chemical potentials, large electrostatic potentials and narrow barriers the temperature generates an increase and broadening in all thermoelectric properties. In the zigzag direction, the figure of merit attains values above the technological limit value $ZT=3$, owing to the significant reduction of the ratio of conductances with respect to the Wiedemann-Franz law and the large values of the Seebeck coefficient. On the contrary, for low chemical potentials, small electrostatic potentials and wide barriers the thermoelectric properties diminish with the temperature. In this case, the figure of merit is dictated essentially by the Seebeck coefficient due to the slight variation of the ratio of conductances with respect to the Wiedemann-Franz law.

How to characterize the electron-vibration interactions at the single-molecule level is a fundamental challenge of molecular electronics. Electron tunneling spectroscopy has so far emerged as one of the powerful tools for witnessing the vibration-mediated electron transport process, while the fluctuation properties of nonthermal vibrations cannot be revealed by simple transport signatures. Here, we study the fluctuation properties of nonthermal vibrations in a current-carrying single molecular junction with or without an external laser radiation. Without the laser drive, the fluctuations of the nonthermal vibrations with sub- and super-Poissonian statistics remain above the zero-point level. When the laser field is turned on, the nonthermal vibrations can be squeezed due to the presence of molecular coherence, where the fluctuations are shown to be reduced below the zero-point level. It is demonstrated that the squeezing and the sub- or super-Poissonian vibration statistics do not always go together. We also propose a cavity engineering to detect the predicted vibration squeezing, where the vibrations and cavity photons are entangled indicated by the two-mode squeezing. This enables optical access to vibration fluctuations in single molecular junctions.

The extent to which disorder influences the properties of topological semimetals is relevant to the understanding of topological states and their use in practical applications. Using molecular beam epitaxy, we achieve systematic control of point defect concentrations in the prototypical Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ to gain insight into the role of disorder on electron transport behavior. Using the guiding center diffusion model for linear magnetoresistance, we extract point defect densities as a function of deposition conditions. We find that reducing cadmium defect concentrations by an order of magnitude results in an $2\times$ increase in the magnetoresistance from 450% to 900%. This finding yields important information in the quest to identify the origin of linear magnetoresistance in a wider range of materials.

We study the ground-state phase diagram of the Shastry-Sutherland model by using a variational optimization of the infinite tensor network states, and identify a weakly first-order transition between the plaquette valence bond solid and the antiferromagnetic states. The full plaquette state is found to strongly compete with the empty plaquette ground state, and can be stabilized as the ground state when a staggered ring-exchange interaction preserving the Shastry-Sutherland lattice symmetry is introduced. We propose the triple point where the full plaquette, empty plaquette, and antiferromagnetic phases meet as a deconfined quantum critical point (DQCP). The analysis of susceptibilities provides evidence of an emergent $\text{SO}\left(5\right)$ symmetry at this point. These results shed light on the study of DQCP in quantum magnets and provide a way to understand the proximate DQCP signatures in recent experiments on ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$.

We investigate magnetotransport across an interface between two Weyl semimetals whose Weyl nodes project onto different interface momenta. Such an interface generically hosts Fermi arcs that connect Weyl nodes of identical chirality in different Weyl semimetals (homochiral connectivity)—in contrast to surface Fermi arcs that connect opposite-chirality Weyl nodes within the same Weyl semimetal (heterochiral connectivity). We show that electron transport along homochiral-connectivity Fermi arcs, in the presence of a longitudinal magnetic field, results in a universal longitudinal magnetoconductance of $e^2/h$ per magnetic flux quantum. Furthermore, a weak tunnel coupling can result in a close encounter of two homochiral-connectivity Fermi arcs, enabling magnetic breakdown. Above the breakdown field the interface Fermi arc connectivity is effectively heterochiral, leading to a saturation of the conductance.

The recent prediction that honeycomb lattices of ${\mathrm{Co}}^{2+}$ $\left(3d^7\right)$ ions could host dominant Kitaev interactions provides an exciting direction for exploration of new routes to stabilizing Kitaev's quantum spin liquid in real materials. ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ has been singled out as a potential material candidate provided that spin and orbital moments couple into a $J_{\mathrm{eff}}=\frac{1}{2}$ ground state, and that the relative strength of trigonal crystal field and spin-orbit coupling acting on Co ions can be tailored. Using x-ray linear dichroism (XLD) and x-ray magnetic circular dichroism (XMCD) experiments, alongside configuration interaction calculations, we confirm the counterintuitive positive sign of the trigonal crystal field acting on ${\mathrm{Co}}^{2+}$ ions and test the validity of the $J_{\mathrm{eff}}=\frac{1}{2}$ description of the electronic ground state. The results lend experimental support to recent theoretical predictions that a compression (elongation) of ${\mathrm{CoO}}_6$ octahedra along (perpendicular to) the trigonal axis would drive this cobaltate toward the Kitaev limit, assuming the $J_{\mathrm{eff}}=\frac{1}{2}$ character of the electronic ground state is preserved.

Surface supported lanthanide single-atom magnets (SAMs) with thermal stability and long magnetic lifetime offer the possibility for the usage of atomic magnetic data storage. Theoretically identifying the factors that prohibit the quantum tunneling of magnetization (QTM) and maximize the magnetic anisotropy energy (MAE) are essential to design novel atomic magnets. The density function theory (DFT), the simulated x-ray absorption, and magnetic circular dichroism spectra as well as crystal field (CF) model Hamiltonians were employed to investigate the electronic structure and magnetic properties of early lanthanide adatoms (Ce, Nd) adsorbed on the MgO/Ag(100) surface. We identified that they can be stably positioned on O atom of MgO surface by strong ionic bond as well as a certain covalent bond via the hybridization between $\mathrm{Ce}/\mathrm{Nd}\text{−}5d$ with $\mathrm{O}\text{−}2p$ orbitals. Our DFT calculations with multiplet simulation reveal that the $4f$ occupancies of these adatoms are essentially identical to that of freestanding atoms. The expectation values of spin/orbital magnetic moments are evaluated as $-0.72/2.86$ ${\mu }_B$ for the ground states $|J=5/2,J_z=\pm 5/2\rangle$ of Ce adatom, yet almost quenched values for the ground-state doublet of Nd adatom due to a mixture of $|J=4,J_z=\pm 4\rangle$ states within equal weights. Although the QTM between the degenerate ground states is prohibited for Ce@MgO/Ag(100), the spin-flip excitations from $|J=5/2,J_z=\pm 5/2\rangle$ ground state to $|J=5/2,J_z=\pm 3/2\rangle$ first excited state followed by the subsequent deexcitation reduce the magnetization reversal barrier from $U_{\mathrm{rev}}=4.17$ meV to 2.87 meV. This energy barrier still guarantees a long magnetic lifetime estimated to be 239 s at 10 K. For Nd@MgO/Ag(100), direct QTM between the degenerate states $|J=4,J_z=\pm 4\rangle$ results in the fast magnetization reversal process without energy barrier and leads to magnetic instability.

The magnetic properties of ${\mathrm{Gd}}_2{\mathrm{Co}}_{2-x}{\mathrm{Mn}}_x{\mathrm{O}}_6$ by modulating the relative proportion of Mn are experimentally investigated in this paper. The experimental results indicated that an intrinsic effect of negative magnetization (NM) characterized by $M_{\mathrm{FC}}<0$ under moderate positive cooling fields $H_{\mathrm{cool}}$ was observed in the samples from $x=1.1$ to 1.5. Magnetic field or temperature-assisted switching effects by manipulating the magnetization states from positive to negative and vice versa were realized with high repeatability, stability, and controllability, which exhibit promising applications in spintronic devices. More interestingly, an inverse exchange bias effect was observed below the compensation temperature $T_{\mathrm{comp}}$. The experiment indicates that the inverse nature of exchange bias has the same origin as NM, which is attributed to the opposite alignment of the net ferromagnetic moments to the applied cooling field below the ferrimagnetic compensation point. In this paper, we provide a promising approach to trigger intriguing magnetic properties for future applications in spintronic devices by modulating the relative proportion of magnetic ions.

We present our studies on polycrystalline samples of fluorine doped ${\mathrm{LaMnO}}_3$ (${\mathrm{LaMnO}}_{3-y}{\mathrm{F}}_y$). ${\mathrm{LaMnO}}_{3-y}{\mathrm{F}}_y$ exhibits remarkable magnetic and electrical properties. It shows ferromagnetic and metallic behavior with a high Curie temperature of $\approx 239$ K and a high magnetoresistance of $-64\%$. This drastic change in magnetic properties in comparison to pure ${\mathrm{LaMnO}}_3$ is ascribed to the presence of mixed-valence Mn ions driven by the F-doping at the O-sites, which enables double exchange in ${\mathrm{LaMnO}}_{3-y}{\mathrm{F}}_y$. Furthermore, the resistivity data exhibits two peaks at 239 and 213 K, respectively. Our results point towards the possibility of multiple double exchange hopping paths of two distinct resistances existing simultaneously in the sample below 213 K.

We study the incubation and transition times that characterize the magnetization switching induced by spin-orbit torques in nanomagnets with perpendicular anisotropy. We present a phenomenological model to interpret the dependence of the incubation time on the amplitude of the voltage pulse and assisting magnetic field and estimate the volume of the seed domain that triggers the switching. Our measurements evidence a correlation between the incubation and transition times that is mediated by the temperature variation during the electric pulse. In addition, we discuss the stochastic distributions of the two times in terms of the energy barriers opposing the nucleation and expansion of the seed domain. We propose two models based on the log-normal and gamma functions to account for the different origin of the variability of the incubation and transition times, which are associated with a single nucleation barrier and multiple pinning sites, respectively.

We investigate critical equilibrium and out of equilibrium properties of a ferromagnetic Ising model in one and two dimensions in the presence of long-range interactions, $J_{ij}\propto r^{-(d+\sigma )}$. We implement a local dynamics on a dynamical Lévy lattice, that correctly reproduces the static critical exponents known in the literature, as a function of the interaction parameter $\sigma$. Due to its locality the algorithm can be applied to investigate dynamical properties, of both discrete and continuous long-range models. We consider the relaxation time at the critical temperature and we measure the dynamical exponent $z$ as a function of the decay parameter $\sigma$, highlighting that the onset of the short-range regime for the dynamical critical properties appears to occur at a value of $\sigma$ which differs from the equilibrium one.

In this study, we investigate the weak localization (WL) and weak antilocalization (WAL) effects in twisted bilayer graphene positioned on a hexagonal boron nitride substrate. The bottom graphene layer aligns with the hexagonal boron nitride. The top layer of the system features a Dirac cone with a negligible gap, while the bottom layer possesses a relatively large band gap. With a low concentration of impurities, the quantum correction to conductivity stems from the quantum interference between two time-reversed impurity scattering trajectories. We discover that interlayer scattering significantly contributes to the conductivity correction when the Fermi surface areas of the two valleys at low energy are comparable. A double crossover from WL to WAL and back to WL occurs at a specific range of Fermi energy, which is particularly intriguing.

Dynamic nuclear polarization (DNP) is capable of boosting signals in nuclear magnetic resonance by orders of magnitude by creating out-of-equilibrium nuclear spin polarization. The diffusion of nuclear spin polarization in the vicinity of paramagnetic dopants is a crucial step for DNP and remains yet not well understood. In this paper, we show that the polarization of the electron spin largely controls the rate of proton spin diffusion in a DNP sample at 1.2 K and 7 T; at increasingly high electron polarization, spin diffusion vanishes. We rationalize our results using a 2 nucleus–1 electron model and Lindblad's master equation, which generalizes preexisting models in the literature and qualitatively accounts for the experimental observed spin diffusion dynamics.

An analysis of orbital magnetization in insulators is provided. It is shown that a previously proposed electronic orbital angular-momentum operator generalizes the “modern theory of orbital magnetization” to include nonlocal Hamiltonians. Expressions for magnetic transition dipole moments needed for the calculation of optical rotation and other properties are developed. A variety of issues that arise in this context are critically analyzed. These issues include periodicity of the operators, previously proposed band dispersion terms, and, if and where needed, evaluation of reciprocal space derivatives of orbital coefficients. Our treatment is used to determine the optical rotatory power of insulators employing a formulation that accounts for electric dipole–electric quadrupole (DQ), as well as electric dipole–magnetic dipole, contributions. An implementation in the public crystal program is validated against a model finite system and applied to the $\alpha$-quartz mineral through linear-response time-dependent density functional theory with a hybrid functional. The latter calculations confirmed the importance of DQ terms. Agreement against experiment was only possible with (i) use of a high-quality basis set, (ii) inclusion of a fraction of nonlocal Fock exchange, and (iii) account of orbital-relaxation terms in the calculation of response functions.

Skyrmion bags are composed of an outer skyrmion and arbitrary inner skyrmions, which have recently been observed in bulk chiral magnets, but still remain elusive in magnetic films. Here, we propose a method of creating skyrmion bags in a thin-film nanodisk, which includes three steps. Firstly, the size of outer skyrmion is enlarged by a vertical magnetic field, then inner skyrmions are nucleated at an off-center area by local current injection, and the system is finally reconstructed due to multiple interskyrmion potentials. Thus, skyrmion bags with topological charge up to forty can be created. Simulated Lorentz transmission electron microscopy images are given to facilitate the experimental demonstration. Our proposal is expected to inspire relevant experiments in magnetic films, and pave the way for potential spintronic applications based on skyrmion bags.

We investigate the interlayer coupling between two thin ferromagnetic (F) films mediated by an antiferromagnetic (AF) spacer in F*/AF/F trilayers and show how it transitions between different regimes on changing the AF thickness. Employing layer-selective Kerr magnetometry and ferromagnetic-resonance techniques in a complementary manner enables us to distinguish between three functionally distinct regimes of such ferromagnetic interlayer coupling. The F layers are found to be individually and independently exchange-biased for thick FeMn spacers—the first regime of no interlayer F-F* coupling. F-F* coupling appears on decreasing the FeMn thickness below 9 nm. In this second regime found in structures with 6.0–9.0-nm-thick FeMn spacers, the interlayer coupling exists only in a finite temperature interval just below the effective Néel temperature of the spacer, which is due to magnon-mediated exchange through the thermally softened antiferromagnetic spacer, vanishing at lower temperatures. The third regime, with FeMn thinner than 4 nm, is characterized by a much stronger interlayer coupling in the entire temperature interval, which is attributed to a magnetic-proximity induced ferromagnetic exchange. These experimental results, spanning the key geometrical parameters and thermal regimes of the F*/AF/F nanostructure, complemented by a comprehensive theoretical analysis, should broaden the understanding of the interlayer exchange in magnetic multilayers and potentially be useful for applications in spin thermionics.

The anomalous Hall effect and the closely related polar Kerr effect are among the most direct evidence of chiral Cooper pairing in some superconductors. While it has been known that disorder or multiband pairing is typically needed for these effects to manifest, there is a lack of direct real-space investigation with regard to how disorder impacts the Hall response in both single-band and multiband chiral superconductors. On the basis of chiral superconducting models often adopted for ${\text{Sr}}_2{\text{RuO}}_4$, we study in this work the anomalous Hall effect in the presence of random nonmagnetic impurities on real-space lattices. The single-band chiral $p$-wave $(p_x+i{p}_y)$ calculation qualitatively reproduces the Hall conductivity obtained in previous skew-scattering-type diagrammatic analyses, along with some quantitative difference originating primarily from contributions involving impurity-induced in-gap states. The non-$p$-wave chiral states, such as $d_{x^2-y^2}+i{d}_{xy}$, generically exhibit finite Hall response in the presence of random impurities, in contrast to a conclusion drawn from the aforementioned diagrammatic study. In particular, while pointlike impurities appear to induce minuscule Hall conductivity in non-self-consistent calculations, self-consistency and finite-range impurity potentials can both lead to substantial Hall conductivity. However, the intrinsic Hall conductivity in multiband chiral superconductors, which is related to interband transitions, decreases parametrically as disorder suppresses the superconducting order parameter. In addition, we check that random impurities do not induce anomalous Hall effect in nonchiral but time-reversal symmetry breaking superconducting states the likes of $s+i{d}_{x^2-y^2}$ and $d_{x^2-y^2}+i{g}_{xy(x^2-y^2)}$. We briefly remark on the implications of our results for Kerr effect measurements.