We study the effect of a magnetic insulator [yttrium iron garnet (YIG)] substrate on the spin-transport properties of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/\mathrm{Al}$ nonlocal spin valve (NLSV) devices. The NLSV signal on the YIG substrate is about two to three times lower than that on a nonmagnetic ${\mathrm{SiO}}_2$ substrate, indicating that a significant fraction of the spin current is absorbed at the Al/YIG interface. By measuring the NLSV signal for varying injector-to-detector distances and using a three-dimensional spin-transport model that takes spin-current absorption at the Al/YIG interface into account, we obtain an effective spin-mixing conductance $G_{\uparrow \downarrow }\simeq 5–8\times {10}^{13}{\Omega }^{-1}{\mathrm{m}}^{-2}$. We also observe a small, but clear, modulation of the NLSV signal when rotating the YIG magnetization direction with respect to the fixed spin polarization of the spin accumulation in the Al. Spin relaxation due to thermal magnons or roughness of the YIG surface may be responsible for the observed small modulation of the NLSV signal.

Magnetization, ${}^{31}\mathrm{P}$ nuclear magnetic resonance study, and first-principles electronic structure calculations have been performed in the spin-1 trimer chain compound ${\mathrm{CaNi}}_3\left({\mathrm{P}}_2{\mathrm{O}}_7\right){}_2$. Two separate spectra arising from magnetically and crystallographically inequivalent P sites are observed. In the ordered state, the resonance lines for both the P sites (P1 and P2) are found to be split into two, which is clear microscopic evidence of the development of two-sublattice AFM order below $T_M$. A nonnegligible contribution of ferromagnetic hyperfine field and dipolar field have also been seen in the ordered state. The first-principles calculations show that the intratrimer $\left(J_1\right)$ and intertrimer interactions $\left(J_2\right)$ are of weak ferromagnetic type with the values 2.85 and 1.49 meV, respectively, whereas the interchain interaction $\left(J_3\right)$ is of strong antiferromagnetic type with a value of 5.63 meV. The anisotropy of the imaginary part of dynamical spin susceptibility around $T_M$ along with the exponential decrement of $1/T_1$ below $T_M$ indicate the probable participation of the $\mathrm{Ni}\text{−}3d$ electron's orbital degrees of freedom in the ferrimagnetic transition. The dominance of orbital fluctuations over the spin fluctuations seems to be responsible for showing low value of the binding energy $u$ of the local spin configuration (estimated from local spin models) and an unusually weak exponent in the power-law behavior of $1/T_1$ below 50 K, in the paramagnetic state. Electronic structure calculations also reveal the importance of orbital degrees of freedom of $\mathrm{Ni}\text{−}3d$ moments, which is consistent with our NMR data analysis.

Time differential perturbed $\gamma \gamma$ angular correlation spectroscopy on ${}^{111}\mathrm{Cd}$ nuclei probes inserted in ${\mathrm{ZrZn}}_{1.9}$ is used to measure the magnetic hyperfine fields at Zr and Zn sites and the electric-field gradient $V_{zz}$ at Zn sites as a function of temperature at various pressures and as a function of pressure at the temperature 4 K. Our data indicate that the local magnetic moment of Zr in the magnetically ordered state is substantially larger than its value obtained from the macroscopic measurements. The transformation between paramagnetic and magnetically ordered states is considered as a fluctuation-induced first-order transition. We conclude that ${\mathrm{ZrZn}}_2$ is not a simple ferromagnet and discuss a possible type of its magnetic ordering.

We present a microscopic theory of biexcitons in colloidal graphene quantum dots, and we discuss the possibility of a biexciton-exciton cascade generation. Assuming a $p_z$ orbital on each carbon atom, the single-particle properties are described in the tight-binding model. The screened direct, exchange, and scattering matrix elements of the Coulomb matrix are calculated using Slater $p_z$ orbitals. The many-body ground and excited states are constructed as a linear combination of a finite number of electron-hole pair excitations from the Hartree-Fock ground state by exact diagonalization techniques. The exciton and biexciton states are constructed exploiting the degeneracy of the valence- and conduction-band edges. The two degenerate exciton $\left(X\right)$ states and a corresponding biexciton $\left(XX\right)$ state are identified for generation of the $XX\text{−}X$ cascade in threefold-symmetric quantum dots. Finally, the Auger coupling of the $XX$ state with the excited $X$ states is predicted.

In this article, we put forward a resolution to the prolonged ambiguity in energy band gaps between theory and experiments of fabricated graphene nanoribbons (GNRs). Band structure calculations using density functional theory are performed on oxygen-passivated GNR supercells of customized edge configurations without disturbing the inherent $s{p}^2$ hybridization of carbon atoms. Direct band gaps are observed for both zigzag and armchair GNRs, consistent with the experimental reports. In addition, we provide an explanation of the experimentally observed scattered band gap values of GNRs as a function of width in a crystallographic orientation on the basis of edge configurations. We conclude that edge configurations of GNRs significantly contribute to band gap formation in addition to its width for a given crystallographic orientation and will play a crucial role in band gap engineering of GNRs for future research on fabrication of nanoelectronic devices.

We study the nonequilibrium Keldysh Green's function for an $N$-orbital Anderson model at high bias voltages, extending a previous work which for the case only with the spin degrees of freedom $N=2$ to arbitrary $N$. Our approach uses an effective non-Hermitian Hamiltonian that is defined with respect to a Liouville-Fock space in the context of a thermal field theory. The result correctly captures the relaxation processes at high energies, and is asymptotically exact not only in the high bias limit, but also in the high-temperature limit at thermal equilibrium. We also present an explicit continued-fraction representation of the Green's function. It clearly shows that the imaginary part is recursively determined by the decay rate of intermediate states with at most $N-1$ particle-hole-pair excitations. These high bias properties follow from the conservations of a generalized charge and current in the Liouville-Fock space. We also examine temperature dependence of the spectral function in equilibrium, comparing the exact results with the numerical finite-$T$ and analytical $T\to \infty$ results of the noncrossing approximation.

We study the transmission and reflection of a plane electromagnetic wave through a two-dimensional array of rf-SQUIDs. The basic equations describing the amplitudes of the magnetic field and current in the split-ring resonators are developed. These yield in the linear approximation the reflection and transmission coefficients. The polarization of the reflected wave is independent of the frequency of the incident wave and of its polarization; it is defined only by the orientation of the split ring. The reflection and transmission coefficients have a strong resonance that is determined by the parameters of the rf-SQUID; its strength depends essentially on the incident angle.

We observe using ab initio methods that localized surface plasmon resonances in icosahedral silver nanoparticles enter the asymptotic region already between diameters of 1 and 2 nm, converging close to the classical quasistatic limit around 3.4 eV. We base the observation on time-dependent density-functional theory simulations of the icosahedral silver clusters ${\mathrm{Ag}}_{55}\left(1.06\mathrm{nm}\right)$, ${\mathrm{Ag}}_{147}\left(1.60\mathrm{nm}\right)$, ${\mathrm{Ag}}_{309}\left(2.14\mathrm{nm}\right)$, and ${\mathrm{Ag}}_{561}(2.68 \mathrm{nm})$. The simulation method combines the adiabatic GLLB–SC exchange-correlation functional with real time propagation in an atomic orbital basis set using the projector-augmented wave method. The method has been implemented for the electron structure code GPAW within the scope of this work. We obtain good agreement with experimental data and modeled results, including photoemission and plasmon resonance. Moreover, we can extrapolate the ab initio results to the classical quasistatically modeled icosahedral clusters.

Magnetoplasmonics provides unique possibilities for magnetic field control over light flow. A special interest in this field is attracted to magnetoplasmonic crystals (MPCs) that are nanostructured metal-dielectric materials with rich resonant spectra that can be tuned by a magnetic field. We show that MPC demonstrates enhanced nonlinear-optical effects such as optical second harmonic generation (SHG) and a giant nonlinear magneto-optical effect. Contrary to the linear-optical case, the main mechanism underlying the observed effects is the resonant enhancement of the local field achieved both for the surface-plasmon-polariton modes at metal/dielectric interfaces and for the waveguide modes in a dielectric slab, which provides an up to 25% magnetic field controlled SHG intensity variation. These effects are treated as an interplay of excitation of different modes that leads to Fano-type resonances in the nonlinear-optical response.

We examine the role of Coulomb interactions in the emergence of macroscopically ordered states in graphene supported on hexagonal boron nitride substrates. Due to incommensuration effects with the substrate and interactions, graphene can develop gapped low-energy modes that spatially conform into a triangular superlattice of quantum rings. In the presence of these modes, we show that Coulomb interactions lead to spontaneous formation of chiral loop currents in bulk and to macroscopic spin-valley order at zero temperature. We show that this exotic state breaks time-reversal symmetry and can be detected with interferometry and polar Kerr measurements.

The electronic structures of a charge density wave (CDW) system $R{\mathrm{Te}}_2$ $(R=\text{Ce},\text{Pr})$ have been investigated by employing angle-resolved photoemission spectroscopy (ARPES) and the first-principles band structure method. The $R$ $4f$ hybridization peak $\left(4f^n{c}^{m-1}\right)$ in the $R$ $4f$ PES spectrum is located deeper in ${\mathrm{PrTe}}_2$ than in ${\mathrm{CeTe}}_2$ and $R$ $4f$ spectral intensity near $E_F$ is much weaker in ${\mathrm{PrTe}}_2$ than in ${\mathrm{CeTe}}_2$, implying the importance of the hybridization between Ce $4f$ and Te(1) $5p$ electrons. For both ${\mathrm{CeTe}}_2$ and ${\mathrm{PrTe}}_2$, the metallic states crossing the Fermi level $\left(E_F\right)$ are observed below the CDW transition temperature, indicating the existence of the partially ungapped Fermi surfaces (FSs). The zigzag features having the fourfold rotational symmetry are observed near the $X$ point in the FS of ${\mathrm{CeTe}}_2$, but not in the FS of ${\mathrm{PrTe}}_2$. The tight-binding model calculations show that the zigzag FS features in ${\mathrm{CeTe}}_2$ can be described as the CDW-induced FS reconstruction due to the $4\times 4$ CDW supercell structure. The effect of the linear dichroism is observed in ARPES, suggesting that the $E_F$-crossing states have mainly the in-plane orbital character. The photon-energy maps for the near-$E_F$ states exhibit the straight vertical dispersions for both ${\mathrm{CeTe}}_2$ and ${\mathrm{PrTe}}_2$, demonstrating the dominant two-dimensional character in $R{\mathrm{Te}}_2$ $(R=\text{Ce},\text{Pr})$.

We study fermions on a triangular lattice model that exhibits topological flatbands characterized by nonzero Chern numbers. Our scheme stems from the well-known Hofstadter model but the next-nearest-neighbor hopping is introduced, which is crucial for tuning the lowest band to be nearly flat. Differing from previous proposals with the necessity of multiparticle interactions, we consider the more realistic long-range dipolar interaction combined with two-body short-range attractions between fermions. We show the realization of the non-Abelian $\nu =1/2$ Moore-Read fractional Chern insulators, and strong evidence for the existence of the more exotic $\nu =3/5$ Read-Rezayi fractional Chern insulators. Our results provide insights for the experimental realization of these exotic states by realistic two-body interactions and thus facilitates the implementation of the universal topological quantum computation.

Due to the confinement effect, the donor wave functions in nanostructures are highly localized on the defect and can even be deformed by the local geometry of the system. This can have relevant consequences on the hyperfine structure of the defect that can be exploited for advanced electronic applications. In this work we employ ab initio density functional calculations to explore the hyperfine structure of S and Se substitutional defects in silicon and germanium nanowires. We show that, if the tetrahedral symmetry is preserved, the hyperfine contact term is only marginally dependent on the nanowire orientation, while it can undergo drastic changes if the symmetry is lost. In addition, we provide an analysis of the strain dependence of the hyperfine structure for the different orientations of the nanowires.

The layered compound $\alpha -{\mathrm{RuCl}}_3$ is composed of a honeycomb lattice of magnetic ${\mathrm{Ru}}^{3+}$ ions with the $4d^5$ electronic state. We have investigated the magnetic properties of $\alpha -{\mathrm{RuCl}}_3$ via magnetization and specific heat measurements using single crystals. It was observed that $\alpha -{\mathrm{RuCl}}_3$ undergoes a structural phase transition at $T_t\simeq 150$ K accompanied by fairly large hysteresis. This structural phase transition is expected to be similar to that observed in closely related ${\mathrm{CrCl}}_3$. The magnetizations and magnetic susceptibilities are strongly anisotropic, which mainly arise from the anisotropic $g$ factors, i.e., $g_{ab}\simeq 2.5$ and $g_c\simeq 0.4$ for magnetic fields parallel and perpendicular to the $ab$ plane, respectively. These $g$ factors and the obtained entropy indicate that the effective spin of ${\mathrm{Ru}}^{3+}$ is one-half, which results from the low-spin state. Specific heat data show that magnetic ordering occurs in four steps at zero magnetic field. The successive magnetic phase transitions should be ascribed to the competition among exchange interactions. The magnetic phase diagram for $H\parallel ab$ is obtained. We discuss the strongly anisotropic $g$ factors in $\alpha -{\mathrm{RuCl}}_3$ and deduce that the exchange interaction is strongly XY-like. $\alpha -{\mathrm{RuCl}}_3$ is magnetically described as a three-dimensionally coupled XY-like frustrated magnet on a honeycomb lattice.

We propose a mechanism that helps stabilize a superconducting state with broken time-reversal symmetry, which was predicted to be realized in a $d$-wave superconducting film [A. B. Vorontsov, Phys. Rev. Lett. 102, 177001 (2009)]. In this superconducting phase, the time-reversal symmetry breaking is accompanied by spontaneous breaking of the translation symmetry along the film surface. We examine how the normal-superconducting phase boundary in the thickness-temperature phase diagram of the film is modified, depending on the Fermi surface shape. In particular, the nonuniform superconducting phase is found to substantially extend to a smaller thickness region in the phase diagram when the Fermi surface satisfies a nesting condition. We demonstrate this Fermi surface effect using a square-lattice tight-binding model.

Besides the conventional bosons and fermions, in synthetic two-dimensional (2D) materials there could exist more exotic quasiparticles with non-Abelian statistics, meaning that the quantum states in the system will be transformed by noncommuting unitary operators when we adiabatically braid the particles one around another. Here, we propose an experimental scheme to observe non-Abelian statistics with cold atoms in a 2D optical lattice. We show that the Majorana-Schockley modes associated with line defects can be braided with non-Abelian statistics through adiabatic shift of the local potentials. We also demonstrate that the braiding operations are robust against typical experimental imperfections and the readout of topological qubits can be accomplished by local measurement of the atom number.

We report the results of our investigation of ${\mathrm{SrPt}}_3\mathrm{P}$, a recently discovered strong-coupling superconductor with $T_c=8.4$ K, by application of high physical pressure and by chemical doping. We study hole-doped ${\mathrm{SrPt}}_3\mathrm{P}$, which was theoretically predicted to have a higher $T_c$, resistively, magnetically, and calorimetrically. Here we present the results of these studies and discuss their implications.