Bipartite fluctuations can provide interesting information about entanglement properties and correlations in many-body quantum systems. We address such fluctuations in relation with the topology of Dirac and Weyl quantum systems, in situations where the relevant particle number is not conserved, leading to additional volume laws scaling with the quantum Fisher information. In the case of the $p+ip$ superconductor, we build a relation between charge fluctuations and the associated winding numbers of Dirac cones in the low-energy sector. Topological aspects of the Hamiltonian in the vicinity of these points induce long-range entanglement in real space. We specifically extract analytical expressions for the corner contributions to bipartite fluctuations. We highlight their differences and similarities with the corner functions of the entanglement entropy which characterize the underlying conformal field theory. We provide a detailed analysis of such fluctuation properties, including the role of gap anisotropy, and discuss higher-dimensional Weyl analogs.

We analyze theoretically nuclear magnetic resonance data for the spin-density-wave phase in the ruthenium-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. Since inhomogeneous distribution of Ru atoms introduces disorder into the system, experimentally observable random spatial variations of the spin-density-wave order parameter emerge. Using perturbation theory for the Landau functional, we estimate the disorder-induced correction to the Néel temperature for this material. Calculated correction is significantly smaller than the Néel temperature itself for all experimentally relevant doping levels. This implies that, despite pronounced spatial nonuniformity of the order parameter, the Néel temperature is quite insensitive to the disorder created by the dopants.

Although cuprate high-temperature superconductors were discovered already in 1986 the origin of the pairing mechanism remains elusive. While the doped compounds are superconducting with high transition temperatures $T_c$, the undoped compounds are insulating due to the strong effective Coulomb interaction between the Cu $3d$ holes. We investigate the dependence of the maximum superconducting transition temperature $T_{c\text{max}}$ on the on-site effective Coulomb interaction $U$ using the constrained random-phase approximation. We focus on the commonly used one-band model of the cuprates, including only the antibonding combination of the Cu $d_{x^2-y^2}$ and O $p_x$ and $p_y$ orbitals and find a screening-dependent trend between the static value of $U$ and $T_{c\text{max}}$ for the parent compounds of a large number of hole-doped cuprates. Our results suggest that superconductivity may be favored by a large on-site Coulomb repulsion. We analyze both the trend in the static value of $U$ and its frequency dependence in detail and, by comparing our results to other works, speculate on the mechanisms behind the trend.

Finding a new class of materials supporting a long spin lifetime is essential in development of energy-saving spintronics, which is achievable by using persistent spin helix (PSH) materials. However, for spintronic devices, the PSH states with large spin splitting are required for operation at room temperature. By employing first-principles calculations, we show that the PSH states with large spin splitting are achieved in the SnSe monolayer functionalized by a substitutional halogen impurity. We find the PSH states in the Fermi level where $k$-space Fermi surface is characterized by the shifted two loops, dominated by out-of-plane spin orientations. We clarify the PSH states in terms of an effective $\vec{k}·\vec{p}$ Hamiltonian obtained from symmetry consideration. Finally, large spin-orbit strength in the PSH states with a substantially small wavelength is found, rendering that this system is promising for the development of efficient and high-density scalable spintronic devices operating at room temperatures.

We present systematic theoretical studies of both bulk and surface electromagnetic eigenmodes, or polaritons, in Weyl semimetals in the minimal model of two bands with two separated Weyl nodes. We derive the tensors of bulk and surface conductivity taking into account all possible combinations of the optical transitions involving bulk and surface electron states. We show how information about electronic structure of Weyl semimetals, such as the position and separation of Weyl nodes, Fermi energy, and Fermi arc surface states, can be unambiguously extracted from measurements of the dispersion, transmission, reflection, and polarization of electromagnetic waves.

In this work, we solve analytically the wave function of the surface states in a generic continuous model describing two-node type-I and -II Weyl and Dirac semimetals or single nodal-line type-I and -II semimetals within a slab geometry. Depending on the values of the parameters, different types of Fermi arcs and drumhead states appear. When the mass terms are dominant with respect to the Fermi velocity in the Hamiltonian, the decay of the surface states becomes oscillatory. This property has important consequences in the stability of surface states in a slab geometry. This exact solution can be used for a better understanding of the behavior of Fermi arcs in real materials and their influence in transport and optical properties. We use these solutions to study the joint density of states at the surface which can be used to interpret quasiparticle interference data in scanning tunneling microscope experiments. We show that oscillatory decay can be distinguished from simple exponential decay of the surface states in these experiments.

Discovering peak-performing components, under certain structural and material constraints, is vital for the efficient operation of integrated systems that incorporate them. This becomes feasible by comprehensive scanning of the parametric space for one of the simplest classes of three-dimensional particles used in visible-light metasurface applications: the core-shell nanosphere. For each combination of actual media picked from a long list, the highest-scoring nanoparticles in terms of absorbance, scattering, and cloaking are recorded, while their near-field visualizations unveil the resonance mechanisms that make them so special. The reported results offer additional degrees of freedom in modeling collective meta-atom interactions and contribute to the photonic inverse design by providing the upper limits in the performance for particles of a basic geometry.

By performing density functional theory and Green's functions calculations, complemented by x-ray photoemission spectroscopy, we investigate the electronic structure of Fe/GeTe(111), a prototypical ferromagnetic/Rashba-ferroelectric interface. We reveal that such a system exhibits several intriguing properties resulting from the complex interplay of exchange interaction, electric polarization, and spin-orbit coupling. Despite a rather strong interfacial hybridization between Fe and GeTe bands, resulting in a complete suppression of the surface states of the latter, the bulk Rashba bands are hardly altered by the ferromagnetic overlayer. This could have a deep impact on spin-dependent phenomena observed at this interface, such as spin-to-charge interconversion, which are likely to involve bulk rather than surface Rashba states.

We use time-resolved detection of the Hanle effect and polarized photoluminescence with dark intervals to investigate the buildup and decay of the spin polarization of nuclei interacting with donor-bound electrons in $n$-doped GaAs. Strong hyperfine coupling defines the millisecond time scale of the spin dynamics of these nuclei, as distinct from the nuclei far from impurity centers, characterized by a thousand times longer spin-relaxation time. The dynamics of spin polarization and relaxation attributed to the nuclei inside the donor orbit is observed on the time scale from 200 to 425 ms.

We develop a multipolar theory of second-harmonic generation (SHG) by dielectric nanoparticles made of noncentrosymmetric materials with bulk quadratic nonlinearity. We specifically analyze two regimes of optical excitation: illumination by a plane wave and single-mode excitation, when the laser pump drives the magnetic dipole mode only. Considering two classes of nonlinear crystalline solids (dielectric perovskite material and III-V semiconductor), we apply a symmetry approach to derive selection rules for the multipolar composition of the nonlinear radiation. The developed description can be used for design of efficient nonlinear optical nanoantennas with reconfigurable radiation characteristics.

In crystalline systems, charge polarization is related to the Zak phase determined by bulk band topology. Nontrivial charge polarization induces robust edge states accompanied by fractional charge. In the Su-Schrieffer-Heeger model, it is known that the strong modulation of electron hopping causes nontrivial charge polarization even in the presence of inversion symmetry. Here, we consider a biatomic honeycomb lattice to introduce such strong modulation, i.e., a ${\mathrm{A}}_3\mathrm{B}$ sheet. By tuning the hopping ratio and on-site potential difference between the A and B atoms, we show that the topological phase transition characterized by the Zak phase occurs. Furthermore, we propose that ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{BC}}_3$ are possible realistic materials on the basis of first-principles calculations. Both of them display topological edge states induced by the Zak phase without spin-orbital couplings and external fields, unlike conventional topological insulators.

The electronic structure of few-layer (FL) black phosphorus (BP) sensitively depends on the sample thickness, strain, and doping. We show that it is also vulnerable to air exposure. The oxidation of BP caused by air exposure gives several optical signatures, including the broadening of resonance peaks and increased Stokes shift between infrared (IR) absorption and photoluminescence (PL) peaks. More importantly, air exposure causes blueshifts of all resonance peaks in IR absorption and PL spectra, with more prominent effects for thinner samples and higher-order subband transitions. Our study alludes to a convenient and exotic way for band-structure engineering of FL-BP through controllable air exposure or defect creation.

We discuss and qualify the connection between two separate phenomena in the physics of nanoscale friction, general in nature and relevant to experiments. The first is thermally assisted lubricity (TAL), i.e., the low-velocity regime where a nanosized dry slider exhibits a viscouslike friction despite corrugations that would otherwise imply hard stick–slip friction. The second is the occurrence of negative dissipated work (NDW) events in sampling the work probability distribution. The abundance, or scarcity due to insufficient sampling, of these NDW events implies experimental fulfillment or violation of the celebrated Jarzynski equality (JE) of nonequilibrium statistical mechanics. We show, both analytically and in simulations of the one-dimensional point slider Prandtl-Tomlinson model, that a general crossover can be individuated as the total frictional work per cycle crosses $k_{B}T$. Below such crossover, the TAL regime holds, the dissipation is essentially linear, and the numerical validation for the JE is feasible (i.e., does not require an exponentially large sampling size). Above it, the dissipation profile departs from linearity and gains its hard stick–slip features, and the mandatory sampling for the JE becomes exponentially large. In addition, we derive a parameter-free formula expressing the linear velocity coefficient of viscous friction, correcting previous empirically parameterized expressions. With due caution, the connection between friction and work tails can be extended beyond a single degree of freedom to more complex sliders, thus inviting realistic crosscheck experiments. Of importance for experimental nanofriction will be the search for NDW tails in the sliding behavior of trapped cold ions, and alternatively checking for TAL in the sliding pattern of dragged colloid monolayers as well as in forced protein unwinding.

We investigate the nonlocal thermoelectric transport in a Cooper-pair splitter based on a double-quantum-dot-superconductor three-terminal hybrid structure. We find that the nonlocal coupling between the superconductor and the quantum dots gives rise to nonlocal thermoelectric effects which originate from the nonlocal particle-hole breaking of the system. We show that Cooper-pair splitting induces the generation of a thermocurrent in the superconducting lead without any transfer of charge between the two normal-metal leads. Conversely, we show that a nonlocal heat exchange between the normal leads is mediated by nonlocal Andreev reflection. We discuss the influence of finite Coulomb interaction and study under which conditions nonlocal power generation becomes possible, and when the Cooper-pair splitter can be employed as a cooling device.

Recent progress in the fabrication techniques of superlattices (SLs) has made it possible to sandwich several-layer-thick block layers (BLs) of heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$ between conventional-metal ${\mathrm{YbCoIn}}_5$ BLs or spin-density-wave-metal ${\mathrm{CeRhIn}}_5$ BLs of a similar thickness. However, the magnetic state in each BL, particularly at the interface, is not yet understood, as experimental techniques applicable to the SL system are limited. Here, we report measurements of ${}^{59}\mathrm{Co}$ nuclear magnetic resonance, which is a microscopic probe of the magnetic properties inside the target BLs. In the ${\mathrm{CeCoIn}}_5/{\mathrm{YbCoIn}}_5$ SL, the low-temperature magnetic fluctuations of the ${\mathrm{CeCoIn}}_5$ BL are weakened as expected from the Rashba spin-orbit effect. However, in the ${\mathrm{CeCoIn}}_5/{\mathrm{CeRhIn}}_5$ SL, the fluctuations show an anomalous enhancement below 6 K, highlighting the importance of the magnetic proximity effect occurring near a magnetic-ordering temperature $T_{\mathrm{N}}\sim 3$ K of the ${\mathrm{CeRhIn}}_5$ BL. We suggest that the magnetic properties of the BLs can be altered by the interfacial interaction, which is an alternative route to modify the magnetic properties.

Two-dimensional (2D) electron systems with a combined Rashba and Dresselhaus spin-orbit interaction (SOI) having a complicated energy spectrum with a conical point and four critical points are promising candidates to observe electron topological transitions. In the present paper we have investigated the evolution of the electron spectrum and isoenergetic contours under the influence of a parallel magnetic field. General formulas for the energies of critical points for arbitrary values of SOI constants and magnetic field are found. The existence of critical magnetic fields at which a number of critical points is changed has been predicted. The magnetic field driving topological Lifshitz transitions in the geometry of isoenergetic contours has been studied. Van Hove's singularities in the electron density of states are calculated. The obtained results can be used for theoretical investigations of the different electron characteristics of such 2D systems.

CrGe belongs to the family of cubic B20 intermetallics. From experimental investigations by susceptibility and de Haas-van Alphen (dHvA) measurements and from calculations of its electronic band structure by density-functional theory (DFT), CrGe is found to form a metallic paramagnetic ground state. Combining dHvA and DFT data, a detailed picture of the Fermi surface of CrGe is provided. The proximity to a magnetic long-range ordering in CrGe is suggested from a prominent thermal magnetic susceptibility. The possibility to induce magnetic long-range order in CrGe is discussed based on calculated properties for CrGe substituting Ge by As or Sn, and from a comparison with MnGe and the alloy series ${\mathrm{Cr}}_{1-x}{\mathrm{Mn}}_x\mathrm{Ge}$. Owing to the noncentrosymmetric and nonsymmorphic crystal structure of CrGe, in absence of broken time reversal symmetry, its band structure is marked by forced nodal lines at the Fermi edge. Moreover, this material hosts degenerate unconventional electronic quasiparticles. In particular, CrGe exhibits a sixfold degeneracy of fermions crossing within about 5 meV of the Fermi energy at the $R$ point of the Brillouin zone.

We study ordered phases with broken translational symmetry in the half-filled three-orbital Hubbard model with antiferromagnetic Hund coupling by means of dynamical mean-field theory (DMFT) and continuous-time quantum Monte Carlo simulations. The stability regions of the antiferro-orbital (AFO), antiferromagnetic (AFM), and charge density wave (CDW) states are determined by measuring the corresponding order parameters. We introduce two symmetrically distinct AFO order parameters and show that these are the primary order parameters in the phase diagram. The CDW and AFM states appear simultaneously with these two types of AFO orders in the weak and strong coupling region, respectively. The DMFT phase diagram is consistent with the results obtained by the Hartree approximation and strong-coupling perturbation theory. In the weak coupling regime, a nontrivial exponent $\beta =3/2$ is found for the CDW order parameter, which is related to the coupling between the CDW and AFO orders in the Landau theory characteristic for the three-orbital model. We also demonstrate the existence of a metallic AFO state without any charge disproportions and magnetic orders, which appears only at finite temperatures.

ZrSiS has recently gained attention due to its unusual electronic properties: nearly perfect electron-hole compensation, large, anisotropic magnetoresistance, multiple Dirac nodes near the Fermi level, and an extremely large range of linear dispersion of up to $\sim 2$ eV. We carried out a series of high pressure electrical resistivity measurements on single crystals of ZrSiS. Shubnikov-de Haas measurements show two distinct oscillation frequencies. For the smaller orbit, we observe a change in the phase of $\sim 0.5$, which occurs between $0.16–0.5$ GPa. This change in phase is accompanied by an abrupt decrease of the cross-sectional area of this Fermi surface. We attribute this change in phase to a possible topological quantum phase transition. The phase of the larger orbit exhibits a Berry phase of $\pi$ and remains roughly constant up to $\sim 2.3$ GPa. Resistivity measurements to higher pressures show no evidence for pressure-induced superconductivity to at least $\sim 20$ GPa.

We study the interlayer scattering mediated by long-range Coulomb interaction between electrons (density $n)$ and holes $\left(p\right)$ in a double-layer system. The gated device is made of InAs $\left(e\right)$ and InGaSb $\left(h\right)$ quantum wells separated by a AlSb middle barrier such that the interlayer tunneling is negligibly small. By using independent-layer contacts we measure the transport tensors ${\rho }_{xx}$ and ${\rho }_{xy}$ that are solely from the InAs layer, while sweeping $p$ in the InGaSb layer. We find a strongly enhanced resistive scattering signal as the carrier densities approach a total charge neutrality, $n=p$, which cannot be described by the Fermi-liquid theory. Results of data analysis for density, temperature, and magnetic field dependences are consistent with the emergence of excitonic coupling between the two layers, stressing the dominance of Coulomb interaction even in the presence of disorder.