We use the tight-binding method to study spin-dependent exciton effects in zigzag-edged graphene nanoribbons (ZGNRs). Our results show the three spin-triplet excitons are not degenerate in energy due to different charge distributions: Electrons and holes are distributed on the opposite edges of ZGNRs for the triplets with spin projection $s_z=0$, while they are on the same edges with $s_z=1$ or $-1$. We also show that spin-orbit coupling can activate optical absorptions of these spin triplets. These results reveal interesting features of excitons in materials with spin-related electronic structures.

We theoretically study the plasmon modes in double parallel metallic armchair graphene nanoribbons (AGNRs) separated by a distance $L_b$. Starting with a single doped metallic AGNR at zero temperature, we show the plasmon dispersion dependence on the Fermi wave vector. By evaluating the static dielectric function for this ribbon, we find that the usual logarithmic divergence at $q=2k_F$ is absent. This indicates that plasmons in metallic AGNRs might be the most robust charge-density oscillations occurring in quasi-one-dimensional electron systems. We also study the influence of the distance $L_b$ and of the carrier densities on the out-of-phase and in-phase plasmon modes. Finally, we address the intra-intersubband electron-hole transitions in single metallic armchair ribbons and find a strong influence of doping on the intrasubband plasmon modes.

Stress introduces anisotropy in the transport coefficients in materials, affecting diffusion. Using first-principles quantum-mechanical methods for activation barriers of atomic jumps, combined with the extended self-consistent mean-field theory to compute transport coefficients with strain-reduced symmetry, we predict significant stress-induced anisotropy for Si impurity diffusion in nickel. This causes complex spatial- and temperature-dependent fluxes; as an example, the heterogenous strain field of a dislocation creates unusual flow patterns that affect mechanical and segregation behavior.

In this work, the ${}^{55}$Mn spin-echo nuclear magnetic resonance technique is used to systematically analyze the complete substitutional series Co${}_2$Mn${}_{1-x}$Fe${}_x$Si in terms of the resonance frequencies of the satellite structure and the corresponding hyperfine fields of the ${}^{55}$Mn nuclei. The frequency spacing between satellite lines is constant within a compound, but decreases with increasing Fe concentration $x$, which originates in changes in the band structure. These changes are also responsible for the altered resonance frequencies observed for the same third shell environment but corresponding to a different overall Fe concentration $x$. All modifications in the hyperfine field upon substitution of Mn by Fe can be uniquely assigned to an entirely random distribution of Mn and Fe limited to only one (4$b$) out of four fcc sublattices constituting the $L{2}_1$ Heusler structure, demonstrating that the complete substitutional series Co${}_2$Mn${}_{1-x}$Fe${}_x$Si is crystallographically very well defined. This very high degree of order sets the stage for the observation of stable half-metallic ferromagnetism which is typically very sensitive to the structural order.

We report the structural transformation of hexagonal Ba${}_3$YIr${}_2$O${}_9$ to a cubic double perovskite form (stable in ambient conditions) under an applied pressure of 8 GPa at 1273 K. While the ambient pressure synthesized sample undergoes long-range magnetic ordering at $\sim 4$ K, the high-pressure (HP) synthesized sample does not order down to 2 K as evidenced from our susceptibility, heat capacity, and nuclear magnetic resonance (NMR) measurements. Further, for the HP sample, our heat capacity data have the form $\gamma T+\beta T^3$ in the temperature ($T$) range of $2–10$ K with the Sommerfeld coefficient $\gamma =10$ mJ/mol-Ir K${}^2$. The ${}^{89}$Y NMR shift has no $T$ dependence in the range of $4–120$ K and its spin-lattice relaxation rate varies linearly with $T$ in the range of $8–45$ K (above which it is $T$ independent). Resistance measurements of both the samples confirm that they are semiconducting. Our data provide evidence for the formation of a $5d$-based, gapless, quantum spin-liquid in the cubic (HP) phase of Ba${}_3$YIr${}_2$O${}_9$. In this picture, the $\gamma T$ term in the heat capacity and the linear variation of ${}^{89}$Y $1/T_1$ arises from excitations out of a spinon Fermi surface. Our findings lend credence to the theoretical suggestion [Chen, Pereira, and Balents, Phys. Rev. B 82, 174440 (2010)] that strong spin-orbit coupling can enhance quantum fluctuations and lead to a QSL state in the double perovskite lattice.

Thin films (250–4500 Å) of epitaxial UO${}_2$ were produced by reactive sputtering on two different substrate materials: LaAlO${}_3$ and CaF${}_2$. Using the large enhancement present with resonant x-ray scattering using photons at the uranium $M$${}_4$ absorption edge, antiferromagnetic (AF) order was found in all films. The ordering temperature $T$${}_N$ is the same as the bulk, but the films show second-order (continuous) transitions in contrast to the first-order bulk transition. For LaAlO${}_3$-based films, an additional strong diffuse magnetic disorder is observed, which is reminiscent of the second-length scale, associated with structural disorder and/or strain. By using a formulation accounting for the strong absorption and coherent nature of the photons, the energy widths at the U $M$${}_4$ resonances can be related to the thickness of the AF region. The LaAlO${}_3$-based films do not order magnetically over more than $\sim$600 Å, whereas the CaF${}_2$-based film orders throughout. Further, for thicker films ($>$1000 Å) the fitting procedure shows that the AF order is located at the top of the LaAlO${}_3$-based film. This points to the formation in thicker films of a nonmagnetic layer of UO${}_2$ adjacent to the substrate, which may have tetragonal symmetry.

URu${}_2$Si${}_2$ is claimed to be a chiral $d$-wave superconductor with a $k_z(k_x\pm i{k}_y)$ time-reversal symmetry broken orbital component for the Cooper pair wave function, which contains both nodal points and lines of nodes. To study the magnetic response of such an unconventional state through a bulk, thermodynamic probe, we measured the magnetic torque $\tau$ in very high-quality, well-characterized URu${}_2$Si${}_2$ single crystals at high magnetic fields $H$ and at very low temperatures $T$. The magnetization $M\left(H\right)\propto \tau \left(H\right)/H$ of URu${}_2$Si${}_2$, in its superconducting state and for angles within ${15}^{\circ }$ from the $\mathit{ab}$ plane, reveals a change in its sign for $H$ approaching $H_{c2}$: from a clear diamagnetic response dominated by the pinning of vortices to a state with a smaller but “paramagneticlike” hysteretic response which disappears at $H_{c2}$, thus implying that it is intrinsically related to the superconducting state. We argue that this anomalous, angular-dependent behavior is evidence for a time-reversal symmetry broken superconducting state in URu${}_2$Si${}_2$, although not necessarily for the $k_z(k_x\pm i{k}_y)$ state.

We have performed ${}^{75}$As and ${}^{23}$Na nuclear magnetic resonance (NMR) measurements on a single crystal of NaFe${}_{0.9835}$Co${}_{0.0165}$As and found microscopic coexistence of superconductivity with a two-component spin density wave (SDW). Using ${}^{23}$Na NMR we measured the spatial distribution of local magnetic fields. The SDW was found to be incommensurate with a major component having magnetic moment ($\sim 0.2{\mu }_B$/Fe) and a smaller component with magnetic moment ($\sim 0.02{\mu }_B$/Fe). Spin lattice relaxation experiments reveal that this coexistence occurs at a microscopic level.

We study adiabatic pumping at a normal metal/class-$D$ superconductor hybrid interface when superconductivity is induced through the proximity effect in a spin-orbit coupled nanowire in the presence of a tilted Zeeman field. When the induced order parameter in the nanowire is nonuniform, the phase diagram has isolated trivial regions surrounded by topological ones. We show that in this case the pumped charge is quantized in units of the elementary charge $e$ and has a topological nature.

Quantum spin liquids are elusive states of matter that are characterized by a lack of long-range magnetic order, which renders their excitation spectra the key to identifying them. Studying the magnetic response of a class of spin-liquid states with fermionic spinons and broken time reversal symmetry (TRS) on the three-dimensional hyperkagome lattice of Na${}_4$Ir${}_3$O${}_8$, we demonstrate that the spinon Fermi surface has a topological transition as a function of the flux intensity generated by the spinons moving along loops within the unit cell. The spin dynamical structure factor reveals small pockets of gapped regions in energy and momentum existing below the topological transition and nondispersive high-intensity peaks which broaden when the TRS gets broken. These dynamical fingerprints closely track the nature of the spin-liquid ground state of Na${}_4$Ir${}_3$O${}_8$.

The coupled cluster method (CCM) is used to study the zero-temperature properties of a frustrated spin-half ($s=\frac{1}{2}$) $J_1$–$J_2$ Heisenberg antiferromagnet (HAF) on a two-dimensional (2D) chevron-square lattice. On an underlying square lattice, each site of the model has four nearest-neighbor exchange bonds of strength $J_1>0$ and two frustrating next-nearest-neighbor (diagonal) bonds of strength $J_2\equiv \kappa J_1>0$, such that each fundamental square plaquette has only one diagonal bond. The diagonal $J_2$ bonds are arranged in a chevron pattern such that along one of the two basic square axis directions (say, along rows), the $J_2$ bonds are parallel, while along the perpendicular axis direction (say, along columns), alternate $J_2$ bonds are perpendicular to each other, and hence form one-dimensional (1D) chevron chains in this direction. The model thus interpolates smoothly between 2D HAFs on the square ($\kappa =0$) and triangular ($\kappa =1$) lattices, and also extrapolates to disconnected 1D HAF chains ($\kappa \to \infty$). The classical ($s\to \infty$) version of the model has collinear Néel order for $0<\kappa <{\kappa }_{\mathrm{cl}}$ and a form of noncollinear spiral order for ${\kappa }_{\mathrm{cl}}<\kappa <\infty$, where ${\kappa }_{\mathrm{cl}}=\frac{1}{2}$. For the $s=\frac{1}{2}$ model, we use both these classical states, as well as other collinear states not realized as classical ground-state (GS) phases, as CCM reference states, on top of which the multispin-flip configurations resulting from quantum fluctuations are incorporated in a systematic truncation hierarchy, which we carry out to high orders and then extrapolate to the physical limit. At each order we calculate the GS energy, GS magnetic order parameter, and the susceptibilities of the states to various forms of valence-bond crystalline (VBC) order, including plaquette and two different dimer forms. We find strong evidence that the $s=\frac{1}{2}$ model has two quantum critical points, at ${\kappa }_{c_1}\approx 0.72\left(1\right)$ and ${\kappa }_{c_2}\approx 1.5\left(1\right)$, such that the system has Néel order for $0<\kappa <{\kappa }_{c_1}$, a form of spiral order for ${\kappa }_{c_1}<\kappa <{\kappa }_{c_2}$ that includes the correct three-sublattice ${120}^{\circ }$ spin ordering for the triangular-lattice HAF at $\kappa =1$, and parallel-dimer VBC order for ${\kappa }_{c_2}<\kappa <\infty$.

We study the combined effects of spin-orbit interaction, magnetic field, and Coulomb charging on the Josephson current-phase relation, $I\left(\phi \right)$, for a multilevel quantum dot tunnel contacted by two conventional $s$-wave superconductors with phase difference $\phi$. A general model is formulated and analyzed in the cotunneling regime (weak tunnel coupling) and in the deep subgap limit, fully taking into account interaction effects. We determine the conditions for observing a finite anomalous supercurrent $I_a=I(\phi =0)$. For a two-level dot with spin-orbit coupling and arbitrarily weak Zeeman field $B$, we find the onset behavior $I_a\propto \mathrm{sgn}\left(B\right)$ in the presence of interactions, suggesting the incipient spontaneous breakdown of time-reversal symmetry. We also provide conditions for realizing spatially separated (but topologically unprotected) Majorana bound states in a double dot variant of this system. Here, Majoranas are predicted to leave a clear signature in the $2\pi$-periodic current-phase relation.

We use inelastic neutron scattering (INS) spectroscopy to study the magnetic excitations spectra throughout the Brillouin zone in electron-doped iron pnictide superconductors BaFe${}_{2-x}$Ni${}_x$As${}_2$ with $x=0.096,0.15,0.18$. While the $x=0.096$ sample is near optimal superconductivity with $T_c=20$ K and has coexisting static incommensurate magnetic order, the $x=0.15,0.18$ samples are electron overdoped with reduced $T_c$ of 14 and 8 K, respectively, and have no static antiferromagnetic (AF) order. In previous INS work on undoped ($x=0$) and electron optimally doped ($x=0.1$) samples, the effect of electron doping was found to modify spin waves in the parent compound BaFe${}_2$As${}_2$ below $\sim$100 meV and induce a neutron spin resonance at the commensurate AF ordering wave vector that couples with superconductivity. While the new data collected on the $x=0.096$ sample confirm the overall features of the earlier work, our careful temperature dependent study of the resonance reveals that the resonance suddenly changes its $Q$ width below $T_c$ similar to that of the optimally hole-doped iron pnictides Ba${}_{0.67}$K${}_{0.33}$Fe${}_2$As${}_2$. In addition, we establish the dispersion of the resonance and find it to change from commensurate to transversely incommensurate with increasing energy. Upon further electron doping to overdoped iron pnictides with $x=0.15$ and 0.18, the resonance becomes weaker and transversely incommensurate at all energies, while spin excitations above $\sim$100 meV are still not much affected. Our absolute spin excitation intensity measurements throughout the Brillouin zone for $x=0.096,0.15,0.18$ confirm the notion that the low-energy spin excitation coupling with itinerant electron is important for superconductivity in these materials, even though the high-energy spin excitations are weakly doping dependent.

We use the Schwinger-Dyson equations as a starting point to derive renormalization flow equations. We show that Katanin's scheme arises as a simple truncation of these equations. We then give the full renormalization group equations up to third order in the irreducible vertex. Furthermore, we show that to the fifth order, there exists a functional of the self-energy and the irreducible four-point vertex whose saddle point is the solution of Schwinger-Dyson equations.

We investigate the optical emission and decay dynamics of excitons confined in large strain-free GaAs quantum dots grown by droplet epitaxy. From time-resolved measurements combined with a theoretical model we show that droplet-epitaxy quantum dots have a quantum efficiency of about 75% and an oscillator strength between 8 and 10. The quantum dots are found to be fully described by a model for strongly confined excitons, in contrast to the theoretical prediction that excitons in large quantum dots exhibit the so-called giant oscillator strength. We attribute these findings to localized ground-state excitons in potential minima created by material intermixing during growth. We provide further evidence for the strong-confinement regime of excitons by extracting the size of electron and hole wave functions from the phonon-broadened photoluminescence spectra. Furthermore, we explore the temperature dependence of the decay dynamics and, for some quantum dots, observe a pronounced reduction in the effective transition strength with temperature. We quantify and explain these effects as being an intrinsic property of large quantum dots owing to thermal excitation of the ground-state exciton. Our results provide a detailed understanding of the optical properties of large quantum dots in general, and of quantum dots grown by droplet epitaxy in particular.

We theoretically show that an entangled state can be prepared between distant heavy-hole spin qubits by using III–V semiconductor quantum dots (QDs) in cascaded cavities. In this scheme, using quantum state transfer between photons and carrier spins, the local spin-singlet state in a virtually excited trion is stretched into the two separated QDs via a single-photon transmission. We calculate the effective Liouville equation and determine the optimal conditions for entanglement creation. We also discuss the possibility of distributing entangled electron spins in the same manner.

Majorana zero modes in networks of one-dimensional topological superconductors obey non-Abelian braiding statistics. Braiding manipulations can be realized by controlling Coulomb couplings in hybrid Majorana-transmon devices. However, strong disorder may induce accidental Majorana modes, which are expected to have detrimental effects on braiding statistics. Nevertheless, we show that the Coulomb-assisted braiding protocol is efficiently realized also in the presence of accidental modes. The errors occurring during the braiding cycle are small if the couplings of the computational Majorana modes to the accidental ones are much weaker than the maximum Coulomb coupling.

In this work we report a Raman study of bilayer graphene in which one layer is of natural isotope composition and the other layer is enriched by the ${}^{13}$C isotope. We use different laser lines in the visible range in order to provide the resonant Raman profiles and to analyze the dispersion with energy of all the features present in the Raman spectra. From the resonant Raman scattering of the $G$ band we determine the twisting angle and the energy gap of different grain domains in the sample. We also present the intensity profiles for three nondispersive extra peaks, characteristic of rotated graphene superlattices. Additionally, by analyzing the dispersive behavior of the 2$D$ bands we determine the Fermi velocity of sample regions with different rotated angles.