We use the density matrix renormalization group (DMRG) algorithm to study the phase diagram of the spin-$\frac{1}{2}$ Heisenberg model on a honeycomb lattice with first ($J_1$) and second ($J_2$) neighbor antiferromagnetic interactions, where a $Z_2$ spin liquid region has been proposed. By implementing SU(2) symmetry in the DMRG code, we are able to obtain accurate results for long cylinders with a width slightly over 15 lattice spacings and a torus up to the size $N=2\times 6\times 6$. With increasing $J_2$, we find a Néel phase with a vanishing spin gap and a plaquette valence-bond (PVB) phase with a nonzero spin gap. By extrapolating the square of the staggered magnetic moment $m_s^2$ on finite-size cylinders to the thermodynamic limit, we find the Néel order vanishing at $J_2/J_1\simeq 0.22$. For $0.25<J_2/J_1\le 0.35$, we find a possible PVB order, which shows a fast growing PVB decay length with increasing system width. For $0.22<J_2/J_1<0.25$, both spin and dimer orders vanish in the thermodynamic limit, which is consistent with a possible spin liquid phase. We present calculations of the topological entanglement entropy, compare the DMRG results with the variational Monte Carlo, and discuss possible scenarios in the thermodynamic limit for this region.

The next-nearest-neighbor hopping term $t^{\prime }$ determines a magnitude, and, hence, the importance of several phenomena in graphene that include self-doping due to broken bonds and the Klein tunneling, which in the presence of $t^{\prime }$, is no longer perfect. Theoretical estimates for $t^{\prime }$ vary widely, whereas a few existing measurements by using polarization-resolved magnetospectroscopy have found surprisingly large $t^{\prime }$, close to or even exceeding the highest theoretical values. Here, we report dedicated measurements of the density of states in graphene by using high-quality capacitance devices. The density of states exhibits a pronounced electron-hole asymmetry that increases linearly with energy. This behavior yields $t^{\prime }$ ≈ −0.3 eV±15$\%$, in agreement with the high end of theory estimates. We discuss the role of electron-electron interactions in determining $t^{\prime }$ and overview phenomena, which can be influenced by such a large value of $t^{\prime }$.

We present a method of extracting the exchange parameters of the classical Heisenberg model from first-principles calculations of spin-spiral total energies based on density functional theory. The exchange parameters of the transition-metal monoxides MnO and NiO are calculated and used to estimate magnetic properties such as transition temperatures and magnon energies. Furthermore we show how to relate the magnon energies directly to differences in spin-spiral total energies for systems containing an arbitrary number of magnetic sublattices. This provides a comparison between magnon energies using a finite number of exchange parameters and the infinite limit.

The tetragonal compound Sr${}_2$RuO${}_4$ exhibits a chiral $p$-wave superconducting (SC) state of its three $t_{2g}$-type conduction bands. The characteristics of unconventional gap structure are known from experiment, in particular field-angle-resolved specific-heat measurements and from microscopic theories. A rotated extremal structure on the main active SC band with respect to the nodal gaps on the passive bands was concluded. We propose that this gap structure can be further specified by applying the scanning tunneling microscopy quasiparticle interference (QPI) method. We calculate the QPI spectrum within a three-band and chiral three-gap model and give closed analytical expressions. We show that as a function of bias voltage, the chiral three-gap model will lead to characteristic changes in QPI that may be identified and may be used for a more quantitative gap determination of the chiral gap structure.

We report the discovery of superconductivity at 2.3 K in Zr${}_5$Sb${}_3$, the first superconducting member in the large compound family of the Mn${}_5$Si${}_3$-type structure. Transport, magnetic, and calorimetric measurements clearly demonstrate bulk superconductivity for Zr${}_5$Sb${}_3$ and suggest it to be a possible phonon-mediated BCS superconductor with a relatively large density of states at the Fermi level associated with the $d$ electrons of Zr and substantially larger electron-phonon coupling compared to the Sn counterpart compound Zr${}_5$Sn${}_3$ from band structure calculations. More superconductors with even higher transition temperatures are expected to be found in this family of compounds.

To establish the mechanism of unconventional superconductivity in Sr${}_2$RuO${}_4$, a prerequisite is direct information concerning the momentum-space structure of the energy gaps ${\Delta }_i\left(k\right)$, and in particular whether the pairing strength is stronger (“dominant”) on the quasi-one-dimensional ($\alpha$ and $\beta$) or on the quasi-two-dimensional ($\gamma$) Fermi surfaces. We present scanning tunneling microscopy measurements of the density of states spectra in the superconducting state of Sr${}_2$RuO${}_4$ for 0.1$T_c$ < $T$ < $T_c$ and analyze them along with published thermodynamic data using a simple phenomenological model. We show that our observation of a single superconducting gap scale with maximum value $2\Delta \approx 5T_c$ along with a spectral shape indicative of line nodes is consistent, within a weak-coupling model, with magnetically mediated odd-parity superconductivity generated by dominant, near-nodal, Cooper pairing on the α and β bands.

The generation, propagation, and control of spin current have attracted a great deal of attention because of the possibility of realizing dissipation-free information propagation. Although electrical generation of spin current originally made significant advances in spin-current devices, novel spin-current-generation approaches without the application of charge current, such as dynamical methods, have also been gaining importance. However, the low spin-current generation efficiency associated with dynamical methods has impeded further progress towards practical spin devices. Here we show that by introducing a single-crystalline ferromagnetic material, Fe${}_3$Si, pure spin currents can be generated about 20 times more efficiently when using a dynamical method. This achievement paves the way to the development of different spin-based devices.

The phase purity and the lattice dynamics in bulk EuTiO${}_3$ were investigated both microscopically, using x-ray and neutron diffraction, ${}^{151}$Eu-Mössbauer spectroscopy, and ${}^{151}$Eu nuclear inelastic scattering, and macroscopically using calorimetry, resonant ultrasound spectroscopy, and magnetometry. Furthermore, our investigations were corroborated by $ab\mathit{initio}$ theoretical studies. The perovskite symmetry, $Pm\overline{3}m$, is unstable at the $M$- and $R$-points of the Brillouin zone. The lattice instabilities are lifted when the structure relaxes in one of the symmetries: $I4/mcm$, $Imma$, $R\overline{3}c$ with relative relaxation energy around $-25\text{meV}$. Intimate phase analysis confirmed phase purity of our ceramics. A prominent peak in the Eu specific density of phonon states at $11.5\text{meV}$ can be modeled in all candidate symmetries. A stiffening on heating around room temperature is indicative of a phase transition similar to the one observed in SrTiO${}_3$, however, although previous studies reported the structural phase transition to the tetragonal $I4/mcm$ phase our detailed sample purity analysis and thorough structural studies using complementary techniques did not confirm a direct phase transition. Instead, in the same temperature range, Eu delocalization is observed which might explain the lattice dynamical instabilities.

We have measured the NMR of ${}^8$Li${}^{+}$ implanted in a set of thin epitaxial films of Pd. We find a large, negative, strongly temperature-dependent Knight shift $K$ consistent with previous measurements on polycrystalline films. The temperature dependence of the shift exhibits a characteristic deviation from the susceptibility $\chi \left(T\right)$. In particular, at low temperature, $K\left(T\right)$ continues to follow a simple Curie-Weiss dependence. This result provides important insight into the origin of the low-temperature behavior of $\chi \left(T\right)$ in strongly paramagnetic metals. In addition, we find the room temperature shift depends on film thickness, with changes on the order of 20% between films 100 nm and 30 nm thick. We also observe a surface-related resonance in both Au-capped and uncapped films with a small positive shift. These features bear a striking similarity to the Pt NMR line shapes in much smaller Pt particles. However, they seem to originate, not from adsorbed species, but rather in confinement effects on the highly exhange-enhanced Pd $d$ band.

Charge order in cuprate superconductors is a possible source of anomalous electronic properties in the underdoped regime. Intraunit cell charge ordering tendencies point to electronic nematic order involving oxygen orbitals. In this context, we investigate charge instabilities in the Emery model and calculate the charge susceptibility within diagrammatic perturbation theory. In this approach, the onset of charge order is signaled by a divergence of the susceptibility. Our calculations reveal three different kinds of order: a commensurate ($q=0$) nematic order, and two incommensurate nematic phases with modulation wave vectors that are either axial or oriented along the Brillouin zone diagonal. We examine the nematic phase diagram as a function of the filling, the interaction parameters, and the band structure. We also present results for the excitation spectrum near the nematic instability, and show that a soft nematic mode emerges from the particle-hole continuum at the transition. The Fermi surface reconstructions that accompany the modulated nematic phases are discussed with respect to their relevance for magneto-oscillation and photoemission measurements. The modulated nematic phases that emerge from the three-band Emery model are compared to those found previously in one-band models.

We show how Majorana end modes can be generated in a one-dimensional system by varying some of the parameters in the Hamiltonian periodically in time. The specific model we consider is a chain containing spinless electrons with a nearest-neighbor hopping amplitude, a $p$-wave superconducting term, and a chemical potential; this is equivalent to a spin-$\frac{1}{2}$ chain with anisotropic $XY$ couplings between nearest neighbors and a magnetic field applied in the $\widehat{z}$ direction. We show that varying the chemical potential (or magnetic field) periodically in time can produce Majorana modes at the ends of a long chain. We discuss two kinds of periodic driving, periodic $\delta$-function kicks, and a simple harmonic variation with time. We discuss some distinctive features of the end modes such as the inverse participation ratio of their wave functions and their Floquet eigenvalues which are always equal to $\pm 1$ for time-reversal-symmetric systems. For the case of periodic $\delta$-function kicks, we use the effective Hamiltonian of a system with periodic boundary conditions to define two topological invariants. The first invariant is a well-known winding number, while the second invariant has not appeared in the literature before. The second invariant is more powerful in that it always correctly predicts the numbers of end modes with Floquet eigenvalues equal to $+1$ and $-1$, while the first invariant does not. We find that the number of end modes can become very large as the driving frequency decreases. We show that periodic $\delta$-function kicks in the hopping and superconducting terms can also produce end modes. Finally, we study the effect of electron-phonon interactions (which are relevant at finite temperatures) and a random noise in the chemical potential on the Majorana modes.

We present the results of photoluminescence (PL) measurements on a type-II GaSb/GaAs quantum dot/ring sample as a function of temperature (2 to 400 K) and over six orders of magnitude of incident laser excitation power. Optically induced charge depletion (OICD) was seen in both the wetting layer (WL) and quantum dots/rings but with remarkably different temperature dependent behavior. Holes originating from background acceptors migrate out of the WL as the sample temperature is raised to 30 K, while the onset of a blueshift in the PL from quantum rings, signaling their thermally induced charging with holes, is only observed at temperatures above 300 K. The presence of dark dots as a hidden reservoir for acceptor holes at the intermediate temperatures is proposed to explain this anomalous behavior. Due to the deep localization potential of GaSb/GaAs, thermalization of acceptor holes between dark dots and bright rings only occurs above room temperature. A rate equation model is presented which successfully replicates the main features of OICD observed here and in previous reports.

The optical properties of single InGaN/GaN heterostructure nanowires (NWs) with a mean diameter down to 18 nm are investigated. Sharp emission lines originating from the recombination of localized excitons in the InGaN disk layer can be resolved. Excitation-dependent energy shifts, together with spectral diffusions of these emission lines, indicate the presence of a weak quantum confined Stark effect (QCSE) caused by nearby charge fluctuations, rather than the screening of piezoelectric polarizations at the InGaN/GaN interface. The absence of a piezoelectric polarization field is further confirmed by time-resolved photoluminescence measurements. Numerical simulations reveal that the elastic strain relaxation via the NW geometry is marginal and occurs only near the NW sidewalls. Carrier localization preferentially near the periphery of the InGaN disk and alloy intermixing are suggested as possible reasons for the absence of a polarization-induced QCSE in thin InGaN/GaN heterostructure NWs.

The spin Coulomb drag (SCD) effect occurs in materials and devices where charged carriers with different spins exchange momentum via Coulomb scattering. This causes frictional forces between spin-dependent currents that lead to intrinsic dissipation, which may limit spintronics applications. A nonlocal formulation of SCD is developed which is valid for strongly inhomogeneous systems such as nanoscale spintronics devices. This nonlocal formulation of SCD is successfully applied to linewidths of intersubband spin plasmons in semiconductor quantum wells, where experiments have shown that the local approximation fails.

The plasmonic properties of spherelike bcc Na nanoclusters ranging from Na${}_{15}$ to Na${}_{331}$ have been studied by real-time time-dependent local density approximation calculations. The optical absorption spectrum, density response function, and static polarizability are evaluated. It is shown that the effect of the ionic background (ionic species and lattice) of the clusters accounts for the remaining discrepancy in the principal (surface plasmon) absorption peak energy between the experiments and previous calculations based on a jellium background model. The ionic background effect also pushes the critical cluster size where the maximum width of the principal peak occurs from Na${}_{40}$ predicted by the previous jellium model calculations to Na${}_{65}$. In the volume mode clusters (Na${}_{27}$, Na${}_{51}$, Na${}_{65}$, Na${}_{89}$, and Na${}_{113}$) in which the density response function is dominated by an intense volume mode, a multiple absorption peak structure also appears next to the principal peak. In contrast, the surface mode clusters of greater size (Na${}_{169}$, Na${}_{229}$, Na${}_{283}$, and Na${}_{331}$) exhibit a smoother and narrower principal absorption peak because their surface plasmon energy is located well within that of the unperturbed electron-hole transitions, and their density responses already bear resemblance to that of classical Mie theory. Moreover, it is found that the volume plasmon that exists only in finite-size particles gives rise to the long absorption tail in the UV region. This volume plasmon manifests itself in the absorption spectrum even for clusters as large as Na${}_{331}$ with an effective diameter of $\sim$3.0 nm.

We report on conductance fluctuation in quasi-one-dimensional wires made of epitaxial Bi${}_2$Se${}_3$ thin film. We found that this type of fluctuation decreases as the wire length becomes longer and that the amplitude of the fluctuation is well scaled to the coherence, thermal diffusion, and wire lengths, as predicted by conventional universal conductance fluctuation (UCF) theory. Additionally, the amplitude of the fluctuation can be understood to be equivalent to the UCF amplitude of a system with strong spin-orbit interaction and no time-reversal symmetry. These results indicate that the conductance fluctuation in Bi${}_2$Se${}_3$ wires is explainable through UCF theory. This work verifies the scaling relationship of UCF in a system with strong spin-orbit interaction.

We report a purely nonempirical generalized gradient approximation for the noninteracting free energy functional of orbital-free density functional theory obtained via a novel constraint-based parametrization scheme. We use that functional to provide forces for finite-temperature molecular dynamics simulations in the warm dense matter (WDM) regime and demonstrate good-to-excellent agreement with reference Kohn-Sham calculations under WDM conditions at a minuscule fraction of the computational cost of corresponding orbital-based simulations.

We investigate the effect of many-body interactions on the optical absorption spectrum of a charge-tunable quantum dot coupled to a degenerate electron gas. A constructive Fano interference between an indirect path, associated with an intradot exciton generation followed by tunneling, and a direct path, associated with the ionization of a valence-band quantum dot electron, ensures the visibility of the ensuing Fermi-edge singularity despite weak absorption strength. We find good agreement between experiment and renormalization group theory, but only when we generalize the Anderson impurity model to include a static hole and a dynamic dot-electron scattering potential. The latter highlights the fact that an optically active dot acts as a tunable quantum impurity, enabling the investigation of a new dynamic regime of Fermi-edge physics.

Singlet-triplet spin qubits in six-electron double quantum dots, in moderate magnetic fields, can show superior immunity to charge noise. This immunity results from the symmetry of orbitals in the second energy shell of circular quantum dots: singlet and triplet states in this shell have identical charge distributions. Our phase-gate simulations, which include $1/f$ charge noise from fluctuating traps, show that this symmetry is most effectively exploited if the gate operation switches rapidly between sweet spots deep in the (3,3) and (4,2) charge stability regions; fidelities very close to 1 are predicted if subnanosecond switching can be performed.

A general theory of reciprocal unidirectional transmission with tunable sign-switchable refraction and deflection of outgoing wave in structures with broken spatial inversion symmetry is presented. Several coupling scenarios connected with the diffraction-inspired asymmetry in transmission are considered for volumetric structures with one-side corrugations. They illustrate conditions of coexistence of diodelike unidirectional transmission and tunability of deflection and refraction, including change of their sign. Simulation results are presented and analyzed for nonsymmetric structures based on the stacked subwavelength hole array metamaterial. It is shown that one-way coupling can appear within a wide range of frequencies and angles of incidence, a part of which may be used to obtain different signs of deflection. Transmission results and field distributions confirm the possibility of obtaining high-contrast unidirectional transmission that can be tuned by varying incidence angle, while frequency is fixed. A pronounced unidirectional transmission is demonstrated at negative deflection in structures that are only 0.47${\lambda }_0$ thick, whereas a thicker structure is required in order to obtain both signs of deflection in a unidirectional regime.