A minimal Kitaev-Gamma model has been recently investigated to understand various Kitaev systems. In the one-dimensional Kitaev-Gamma chain, an emergent $\mathrm{SU}{\left(2\right)}_1$ phase and a rank-1 spin ordered phase with $O_h\to D_4$ symmetry breaking were identified using non-Abelian bosonization and numerical techniques. However, puzzles near the antiferromagnetic Kitaev region with finite Gamma interaction remained unresolved. Here we focus on this parameter region and find that there are two new phases, namely, a rank-1 ordered phase with an $O_h\to D_3$ symmetry breaking, and a peculiar Kitaev phase. Remarkably, the $O_h\to D_3$ symmetry breaking corresponds to the classical magnetic order, but appears in a region very close to the antiferromagnetic Kitaev point where the quantum fluctuations are presumably very strong. In addition, a two-step symmetry breaking $O_h\to D_{3d}\to D_3$ is numerically observed as the length scale is increased: At short and intermediate length scales, the system behaves as having a rank-2 spin nematic order with $O_h\to D_{3d}$ symmetry breaking; and at long distances, time reversal symmetry is further broken leading to the $O_h\to D_3$ symmetry breaking. Finally, there is no numerical signature of spin orderings nor Luttinger liquid behaviors in the Kitaev phase whose nature is worth further studies.

The magnetic properties of site-disordered ${\mathrm{Ni}}_5\mathrm{Sn}{\left({\mathrm{O}}_2{\mathrm{BO}}_3\right)}_2$ ludwigite have carefully been studied using local probes and bulk experimental methods in temperatures down to 1.6 K and magnetic fields up to 9 T. Our results have clearly shown the role of the effect of order/disorder site occupancy in these complex compounds with spin moments $S=1$. Different from most ludwigites, site-disordered ${\mathrm{Ni}}_5\mathrm{Sn}{\left({\mathrm{O}}_2{\mathrm{BO}}_3\right)}_2$ ludwigite has shown three magnetic transitions at 80, 50, and 5 K. Local probes (${}^{119}\mathrm{Sn}$ Mössbauer and muon spin spectroscopy) have indicated a partial long-range magnetic order with onsets at 80 and 50 K with predominant ferromagnetic interactions at 80 K and antiferromagnetic ones at 50 K. At lower temperatures, a partial spin-glass-like freezing of the moments has been developed concomitantly with fraction of the moments ordered at higher temperatures, a spin-glass-like and long-range magnetic order coexist and a spontaneous exchange bias effect has been observed. Applied magnetic fields have gradually reduced magnetic frustration, leading to a magnetically ordered state of the complete structure at 51 K under 9 T. The absence of double-exchange interactions in order/disorder ${\mathrm{Ni}}_5\mathrm{Sn}{\left({\mathrm{O}}_2{\mathrm{BO}}_3\right)}_2$ ludwigites shows how the magnetic interactions that compete as direct exchange and double exchange are modified by the site occupancy of nonmagnetic Sn ions. Features as a third magnetic transition, the appearance of spontaneous exchange bias, and the enhancement of the coercive field at low temperatures were not found in the site-ordered isomer compound.

Eliashberg's foundational theory of superconductivity is based on the application of Migdal's approximation, which states that vertex corrections to lowest-order electron-phonon scattering are negligible if the ratio between phonon and electron energy scales is small. The resulting theory incorporates the first Feynman diagrams for electron and phonon self-energies. However, the latter is most commonly neglected in numerical analyses. Here we provide an extensive study of full-bandwidth Eliashberg theory in two and three dimensions, where we include the full back reaction of electrons onto the phonon spectrum. We unravel the complex interplay between nesting properties, size of the Fermi surface, renormalized electron-phonon coupling, phonon softening, and superconductivity. We propose furthermore a scaling law for the maximally possible critical temperature $T_c^{\text{max}}\propto \lambda \left(\mathrm{\Omega }\right)\sqrt{{\mathrm{\Omega }}_0^2-{\mathrm{\Omega }}^2}$ in two- and three-dimensional systems, which embodies both the renormalized electron-phonon coupling strength $\lambda \left(\mathrm{\Omega }\right)$ and softened phonon spectrum $\mathrm{\Omega }$. Also, we analyze for which electronic structure properties a maximal $T_c$ enhancement can be achieved.

We study the low-frequency spectra of complex Sachdev-Ye-Kitaev models at general densities. The analysis applies also to $\mathrm{SU}\left(M\right)$ magnets with random exchange at large $M$. The spectral densities are computed by numerical analysis of the saddle-point equations on the real frequency $\omega$ axis at zero temperature $T$. The asymptotic low-$\omega$ behaviors are found to be in excellent agreement with the scaling dimensions of irrelevant operators which perturb the conformally invariant critical states. Of possible experimental interest is our computation of the universal spin spectral weight of the $\mathrm{SU}\left(M\right)$ magnets at low $\omega$ and $T$; this includes a contribution from the time reparametrization mode, which is the boundary graviton of the holographic dual. This analysis is extended to a random $t-J$ model in the following paper [M. Tikhanovskaya et al., following paper, Phys. Rev. B 103, 075142 (2021)].

We present numerical solutions of the spectral functions of large dimension $t\text{−}J$ models with random nearest-neighbor exchange and global $\mathrm{SU}\left(M\right)$ spin rotation symmetry. The solutions are obtained from the saddle-point equations of the large dimension limit, followed by the large $M$ limit. The same saddle point equations also apply to the model with all-to-all and random hopping and exchange. Such a $t\text{−}J$ model realizes a deconfined critical state proposed as a model for the optimally doped cuprates. The large $M$ theory involves Green's functions for fractionalized spinons and holons carrying emergent U(1) gauge charges, obeying relations similar to those of the Sachdev-Ye-Kitaev (SYK) models. The low frequency spectral functions are compared with an analytic analysis of the operator scaling dimensions with good agreement. We also compute the low frequency and temperature behavior of experimentally observable gauge-invariant observables: the electron Green's function, the local spin susceptibility and the optical conductivity, along with the temperature dependence of the d.c. resistivity. The time reparameterization soft mode (equivalent to the boundary graviton in holographically dual models of two-dimensional quantum gravity) makes important contributions to all observables and provides a linear-in-temperature contribution to the d.c. resistivity.

We report a comprehensive study of the electronic structure of layered compound $\mathrm{Ba}\mathrm{Cd}{\mathrm{Sb}}_2$ by using electrical transport measurements, first-principles calculations, and angle-resolved photoemission spectroscopy (ARPES). The samples show semiconductorlike temperature dependence of resistivity and low carrier density. The Shubnikov-de Haas (SdH) oscillations with a single frequency reveal a small two-dimensional (2D) cylinderlike Fermi surface (FS), a light cyclotron effective mass, and trivial Berry's phase. Quantum limit can be achieved under a moderate magnetic field. The calculated bands without any renormalization are well consistent with the ARPES results, showing the Dirac band crossing with 2D character near the Fermi energy and a gap of about 34 meV at the Dirac point induced by spin-orbit interaction. The SdH oscillations can be identified to the Dirac band by the similar cross sectional areas of FSs. Our findings indicate $\mathrm{Ba}\mathrm{Cd}{\mathrm{Sb}}_2$ is a promising material for researching the quantum phenomena of the 2D Dirac fermions.

Spin-orbital liquids are quantum disordered states in systems with entangled spin and orbital degrees of freedom. We study exactly solvable spin-orbital models in two dimensions with selected Heisenberg-, Kitaev-, and $\mathrm{\Gamma }$-type interactions, as well as external magnetic fields. These models realize a variety of spin-orbital-liquid phases featuring dispersing Majorana fermions with Fermi surfaces, nodal Dirac or quadratic band touching points, or full gaps. In particular, we show that Zeeman magnetic fields can stabilize nontrivial flux patterns and induce metamagnetic transitions between states with different topological character. Solvable nearest-neighbor biquadratic spin-orbital perturbations can be tuned to stabilize zero-energy flat bands. We discuss in detail the examples of $\mathrm{SO}\left(2\right)$- and $\mathrm{SO}\left(3\right)$-symmetric spin-orbital models on the square and honeycomb lattices, and use group-theoretical arguments to generalize to $\mathrm{SO}\left(\nu \right)$-symmetric models with arbitrary integer $\nu >1$. These results extend the list of exactly solvable models with spin-orbital-liquid ground states and highlight the intriguing general features of such exotic phases. Our models are thus excellent starting points for more realistic modelling of candidate materials.

The scattering theory for two crossing metallic single-walled carbon nanotubes of finite length exposed to an electromagnetic field has been developed based on a synthesis of the quantum transport formalism and classical electrodynamics. The model of the point contact has been developed to be incorporated into the Hallén and Pocklington equations for crossing carbon nanotubes. The influence of the contact conductance and position as well as the angle between the tube axes on the tube polarizability has been analyzed in the range of 1 GHz to 10 THz. The physical mechanisms responsible for the electromagnetic interaction between crossing tubes with zero and nonzero intertube contact conductance are shown and discussed. The influence of the coupling between the tubes on the localized plasmon resonance in them has been demonstrated.

Josephson junctions in two-dimensional topological insulators with embedded magnetic domains can host a number of topological phases, in particular, Jackiw-Rebbi solitons and Majorana zero modes. These different nontrivial phases appear in such junctions for multiple-domain magnetic islands, showing a rich multigap structure. Features related to the interplay between superconductivity and magnetism in these systems cannot be easily discerned looking at behavior of the Andreev spectrum and the concomitant dc Josephson effect. Instead, the thermal conductance is very sensitive to the nature of the junction and the domain structure of the magnetic island. We present a detailed analysis of these properties in the case of a topological Josephson junction with a single- and two-domain magnetic island. Configurations hosting soliton magnetic modes lead to a peculiar behavior of the thermal conductance relative to the thermal quantum, characterized by a negative slope as a function of the temperature, just above the superconducting critical temperature. At low temperatures, these junctions also show characteristic coherence patterns in the behavior of the thermal conductance as a function of the Josephson phase bias and the angle between the magnetizations of the domains.

We investigate a quantum quench from a critical to an exceptional point. The initial state, prepared in the ground state of a critical Hermitian system, is time evolved with a non-Hermitian SSH model, tuned to its exceptional point. The single particle density matrix exhibits supersonic modes and multiple light cones, characteristic of non-Hermitian time evolution. These propagate with integer multiples of the original Fermi velocity. In the long time limit, the fermionic Green's function decays spatially as $1/x^2$, in sharp contrast to the usual $1/x$ decay of noninteracting fermions. The entanglement entropy is understood as if all these supersonic modes arise from independent quasiparticles (though they do not), traveling with the corresponding supersonic light cone velocity. The entropy production rate decreases with time and develops plateaus during the time evolution, signaling the distinct velocities in the propagation of nonlocal quantum correlations. At late times, the entanglement entropy saturates to a finite value, satisfying a volume law.

Magnetic topological quantum materials have attracted great attention due to their exotic topological quantum physics induced by the interplay among crystalology, magnetism, and topology, which is of profound importance to fundamental research and technology applications. However, limited materials are experimentally available, most of whom are realized by magnetic impurity doping or heterostructural constructions. In this work, based on the first-principles calculations, we predict that double perovskite ${\mathrm{Ba}}_2{\mathrm{CdReO}}_6$ is an intrinsic ferromagnetic topological semi-half-metal, while the ferrimagnetic double perovskite with space group symmetry $Fm\overline{3}m$, such as ${\mathrm{Ba}}_2{\mathrm{FeMoO}}_6$, belongs to a topological half-metal. One pair of Weyl points and fully spin-polarized nodal-ring states are found in the vicinity of the Fermi level in ${\mathrm{Ba}}_2{\mathrm{CdReO}}_6$. Its two-dimensional nearly flat drumhead surface states are fully spin polarized. In ${\mathrm{Ba}}_2{\mathrm{FeMoO}}_6$, however, there exist four pairs of Weyl points and two fully spin-polarized nodal rings near the Fermi level. These topological properties are stable in the presence of spin-orbit coupling. This makes these materials an appropriate platform for studying the emerging intriguing properties, especially for the applications in spintronics, information technology, and topological superconductivity.

We have studied the electron pair emission process from a Cu(111) surface excited by a primary electron or photon beam. Although the photon energy was 30 eV the primary electron beam had an energy of 25 eV. We will discuss the energy and momentum distributions. The experiments with the electron beam demonstrate that the contributions from the Shockley surface state can be clearly identified. This is not the case for the work with photons which we ascribe to the low spectral weight of the contribution from two Shockley state electrons compared to two $3d$ electrons. The momentum distributions reveal a close similarity between the two types of excitation despite the fact that the microscopic origin of the pair emission is very different. There is a distinctive energy dependence of these curves which we describe by an empirical three-parameter polynomial. The numerical values of these parameters are close for both pathways. Additional measurements with different primary electron energies reveals the importance of the initial state on the momentum distributions.

This work aims to provide a simple yet complete effective dielectric function for an anisotropic layer of polarizable molecules adsorbed on a metallic surface. This effective medium model considers the important and nontrivial case of nonvacuum embedding media and accounts for orientation effects, coverage dependence through dipole-dipole interactions, and image-dipole effects. To check the model's validity, we focus in particular on the experimentally relevant case of dyes adsorbed on metallic nanospheres. We can then use anisotropic Mie theory, together with the effective dielectric function describing the molecular coating, to calculate their optical properties. We show that this effective medium description is in very good agreement with more elaborate and computationally intensive microscopic calculations based on coupled-dipole models. The effective medium model therefore provides a simple means to investigate orientation effects and coverage dependence, including in more complex systems such as dyes adsorbed on nonspherical or ensembles of nanoparticles. This model can readily be used to further our theoretical understanding of dye-nanoparticle systems, for example in the context of dye-plasmon resonance coupling or surface-enhanced Raman and fluorescence spectroscopy.

A gate-induced insulating behavior at zero magnetic field is observed in a high-mobility suspended monolayer graphene near the charge neutrality point. The graphene device initially cleaned by a current annealing technique underwent a thermopressure cycle to allow short-range impurities to be adsorbed directly by the ultraclean graphene surface. The adsorption process generated a strong temperature- and electric-field-dependent behavior on the conductance of the graphene device. The conductance around the neutrality point is observed to be reduced from around $e^2/h$ at 30 K to $\sim 0.01e^2/h$ at 20 mK. A direct transition from an insulator to a quantum Hall conductor within $\approx 0.4\text{T}$ accompanied by broken-symmetry-induced $\nu =0,\pm 1$ plateaus confirms the presence of intervalley scatterers.

We show that geometrically coupled polariton condensates fabricated in semiconductor devices are versatile systems capable of simulating molecules with given characteristics. In particular, we consider oscillatory and stationary symmetric and asymmetric states in polariton dimers, trimers, and tetrads and their luminosity in real and Fourier space. The spectral weights of oscillatory states are associated with discrete spectral lines. Their number and separation can be controlled by changing the number and geometry of condensates, reflected by the coupling strengths. We also show that asymmetric stationary states combine discrete and continuous degrees of freedom in one system. The continuous degree of freedom is represented by the phase while the discrete degree of freedom is given by density asymmetry. Our work paves the way to engineer controllable artificial molecules with a range of properties manufactured on demand.

van der Waals heterostructures based on two-dimensional materials have recently become a very active topic of research in spintronics, both aiming at a fundamental description of spin dephasing processes in nanostructures and as a potential element in spin-based information processing schemes. Here, we theoretically investigate the magnetoconductance of mesoscopic devices built from graphene proximity-coupled to a high spin-orbit coupling material. Through numerically exact tight-binding simulations, we show that the interfacial breaking of inversion symmetry generates robust weak antilocalization even when the $z\to -z$ symmetric spin-orbit coupling in the quantum dot dominates over the Bychkov-Rashba interaction. Our findings are interpreted in the light of random matrix theory, which links the observed behavior of quantum interference corrections to a transition from a circular-orthogonal to circular-symplectic ensemble.

By means of density functional theory plus dynamical mean-field theory (DFT $+$ DMFT) calculations and resonant inelastic x-ray scattering (RIXS) experiments, we investigate the high-pressure phases of the spin-orbit-coupled $J_{\mathrm{eff}}=3/2$ insulator ${\mathrm{GaTa}}_4{\mathrm{Se}}_8$. Its metallic phase, derived from the Mott state by applying pressure, is found to carry $J_{\mathrm{eff}}=3/2$ moments. The characteristic excitation peak in the RIXS spectrum maintains its destructive quantum interference of $J_{\mathrm{eff}}$ at the Ta $L_2$ edge up to 10.4 GPa. Our exact diagonalization-based DFT $+$ DMFT calculations including spin-orbit coupling also reveal that the $J_{\mathrm{eff}}=3/2$ character can be clearly identified under high pressure. These results establish the intriguing nature of the correlated metallic magnetic phase, which represents the first confirmed example of $J_{\mathrm{eff}}=3/2$ moments residing in a metal. They also indicate that the pressure-induced superconductivity is likely unconventional and influenced by these $J_{\mathrm{eff}}=3/2$ moments. Based on a self-energy analysis, we furthermore propose the possibility of doping-induced superconductivity related to a spin-freezing crossover.