It is shown that a racemic array of helical-shaped metallic wires may have a dual electromagnetic response, such that for arbitrarily large wavelengths it concurrently supports two modes with hyperbolic- and elliptical-type dispersions. Importantly, one of the eigenwaves is nearly dispersionless and sees the metamaterial as a medium with extreme magnetic anisotropy. The metamaterial may thus behave as the magnetic analog of the conventional wire medium formed by a set of parallel straight metallic wires. It is demonstrated that the magnetic wire medium enables channeling the subwavelength details of transverse electric (TE) polarized waves.

We present a resonant inelastic x-ray scattering (RIXS) study of spin and charge excitations in overdoped ${\mathrm{La}}_{1.77}{\mathrm{Sr}}_{0.23}{\mathrm{CuO}}_4$ along two high-symmetry directions. The line shape of these excitations is analyzed and they are shown to be highly overdamped. Their spectral weight and damping are found to be strongly momentum dependent. Qualitative agreement between these observations and a calculated random-phase approximation susceptibility is obtained for this overdoped compound, implying that a significant contribution to the RIXS signal stems from a continuum of charge excitations. Furthermore, this suggests that the spin excitations in the overdoped regime can be captured qualitatively by an itinerant picture. Our calculations also predict a low-energy spin-excitation branch to exist along the nodal direction near the zone center. With the energy resolution of the present experiment, this branch is not resolvable, but we show that the next generation of high-resolution spectrometers will be able to test this prediction.

The implementation of a functional quantum computer involves entangling and coherent manipulation of a large number of qubits. For qubits based on electron spins confined in quantum dots, which are among the most investigated solid-state qubits at present, architectural challenges are often encountered in the design of quantum circuits attempting to assemble the qubits within the very limited space available. Here, we provide a solution to such challenges based on an approach to realizing entanglement of spin qubits over long distances. We show that long-range Ruderman-Kittel-Kasuya-Yosida interaction of confined electron spins can be established by quantum Hall edge states, leading to an exchange coupling of spin qubits. The coupling is anisotropic and can be either Ising type or XY type, depending on the spin polarization of the edge state. Such a property, combined with the dependence of the electron spin susceptibility on the chirality of the edge state, can be utilized to gain valuable insights into the topological nature of various quantum Hall states.

Control of zero-field spin splitting is realized in a dual-gated high-quality InAs-AlSb two-dimensional electron gas. Magnetotransport experiments showed clean Shubnikov–de Haas oscillations down to low magnetic fields, and the gate-tuned electron mobility exceeded $700000\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$. A clear beating effect was observed in magnetoresistance oscillations at large potential asymmetry between gates. Beat patterns due to zero-field spin splitting and other classes of transverse magnetoresistance oscillations were distinguished by temperature-dependent magnetoresistance measurements. Analysis of the magnetoresistance oscillations indicated that the zero-field spin splitting could be tuned via the Rashba effect while keeping the two-dimensional electron gas charge density constant.

Characterization of strain in two-dimensional (2D) crystals is important for understanding their properties and performance. Using first-principles calculations, we study the effects of uniaxial strain on the Raman-active modes in monolayer ${\text{MoS}}_2$. We show that the in-plane $E^{\prime }$ mode at 384 ${\text{cm}}^{-1}$ and the out-of-plane $A_1^{\prime }$ mode at 403 ${\text{cm}}^{-1}$ can serve as fingerprints for the uniaxial strain in this 2D material. Specifically, under a uniaxial strain, the doubly degenerate $E^{\prime }$ mode splits into two nondegenerate modes: one is the $E_{\parallel }^{\prime }$ mode in which atoms vibrate in parallel to the strain direction, and the other is the $E_{\perp }^{\prime }$ mode in which atoms vibrate perpendicular to the strain direction. The frequency of the $E_{\parallel }^{\prime }$ mode blueshifts for a compressive strain, but redshifts for a tensile strain. In addition, due to the strain-induced anisotropy in the ${\text{MoS}}_2$ lattice, the polarized Raman spectra of the $E_{\parallel }^{\prime }$ and $E_{\perp }^{\prime }$ modes exhibit distinct angular dependence for specific laser polarization setups, allowing for a precise determination of the direction of the uniaxial strain with respect to the crystallographic orientation. Furthermore, we find that the polarized Raman intensity of the $A_1^{\prime }$ mode also shows evident dependence on the applied strain, providing additional effective clues for determining the direction of the strain even without knowledge of the crystallographic orientation. Thus, polarized Raman spectroscopy offers an efficient nondestructive way to characterize the uniaxial strains in monolayer ${\text{MoS}}_2$.

We study the ac charge and -spin transport through an orbital of a magnetic molecule with spin precessing in a constant magnetic field. We assume that the source and drain contacts have time-dependent chemical potentials. We employ the Keldysh nonequilibrium Green's functions method to calculate the spin and charge currents to linear order in the time-dependent potentials. The molecular and electronic spins are coupled via exchange interaction. The time-dependent molecular spin drives inelastic transitions between the molecular quasienergy levels, resulting in a rich structure in the transport characteristics. The time-dependent voltages allow us to reveal the internal precession time scale (the Larmor frequency) by a dc conductance measurement if the ac frequency matches the Larmor frequency. In the low-ac-frequency limit the junction resembles a classical electric circuit. Furthermore, we show that the setup can be used to generate dc-spin currents, which are controlled by the molecular magnetization direction and the relative phases between the Larmor precession and the ac voltage.

We study the interplay between large-spin, spin-orbit coupling, and superfluidity for bosons in a two-dimensional optical lattice, focusing on the spin-1 spin-orbit-coupled system recently realized at the Joint Quantum Institute [Campbell et al., arXiv:1501.05984]. We find a rich quantum phase diagram where, in addition to the conventional phases—superfluid and insulator—contained in the spin-1 Bose-Hubbard model, there are new lattice symmetry breaking phases. For weak interactions, the interplay between two length scales, the lattice momentum and the spin-orbit wave vector, induce a phase transition from a uniform superfluid to a phase where bosons simultaneously condense at the center and edge of the Brillouin zone at a nonzero spin-orbit strength. This state is characterized by spin-density-wave order, which arises from the spin-1 nature of the system. Interactions suppress spin-density-wave order, and favor a superfluid only at the Brillouin zone edge. This state has spatially oscillating mean-field order parameters, but a homogeneous density. We show that the spin-density-wave superfluid phase survives in a two-dimensional harmonic trap, and thus establish that our results are directly applicable to experiments on ${}^{87}\mathrm{Rb},{}^7\mathrm{Li}$, and ${}^{41}\mathrm{K}$.

The dimer Mott insulator $\kappa -\left(\text{BEDT-TTF}{)}_2{\mathrm{Cu}}_2\right(\mathrm{CN}{)}_3$ exhibits unusual electrodynamic properties. Numerical investigations of the electronic ground state and the molecular and lattice vibrations reveal the importance of the ${\mathrm{Cu}}_2(\mathrm{CN}{)}_3{}^{-}$ anion network coupled to the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecules: The threefold cyanide coordination of copper and linkage isomerism in the anion structure cause a loss of symmetry, frustration, disorder, and domain formation. Our findings consistently explain the temperature and polarization-dependent THz and infrared measurements, reinforce the understanding of dielectric properties, and have important implications for the quantum spin-liquid state, which should be treated beyond two-dimensional, purely electronic models.

Controlling the flow of spin and charge currents in topological insulators (TIs) is a crucial requirement for applications in quantum computation and spin electronics. We demonstrate that such control can be established in nanoscopic two-dimensional TIs by breaking their time-reversal symmetry via magnetic defects. This allows for the creation of nearly fully spin-polarized charge currents, and the design of highly tunable spin diodes. Similar effects can also be realized in mesoscale hybrid structures in which TIs interface with ferro- or antiferromagnets.

The uniform electron gas (UEG) at finite temperature is of key relevance for many applications in the warm dense matter regime, e.g., dense plasmas and laser excited solids. Also, the quality of density functional theory calculations crucially relies on the availability of accurate data for the exchange-correlation energy. Recently, results for $N=33$ spin-polarized electrons at high density, $r_s=\overline{r}/a_B\lesssim 4$, and low temperature have been obtained with the configuration path integral Monte Carlo (CPIMC) method [T. Schoof et al., Phys. Rev. Lett. 115, 130402 (2015)]. To achieve these results, the original CPIMC algorithm [T. Schoof et al., Contrib. Plasma Phys. 51, 687 (2011)] had to be further optimized to cope with the fermion sign problem (FSP). It is the purpose of this paper to give detailed information on the manifestation of the FSP in CPIMC simulations of the UEG and to demonstrate how it can be turned into a controllable convergence problem. In addition, we present new thermodynamic results for higher temperatures. Finally, to overcome the limitations of CPIMC towards strong coupling, we invoke an independent method—the recently developed permutation blocking path integral Monte Carlo approach [T. Dornheim et al., J. Chem. Phys. 143, 204101 (2015)]. The combination of both approaches is able to yield ab initio data for the UEG over the entire density range, above a temperature of about one half of the Fermi temperature. Comparison with restricted path integral Monte Carlo data [E. W. Brown et al., Phys. Rev. Lett. 110, 146405 (2013)] allows us to quantify the systematic error arising from the free particle nodes.

Progress in the understanding of quantum critical properties of itinerant electrons has been hindered by the lack of effective models which are amenable to controlled analytical and numerically exact calculations. Here we establish that the disorder-driven semimetal to metal quantum phase transition of three-dimensional massless Dirac fermions could serve as a paradigmatic toy model for studying itinerant quantum criticality, which is solved in this work by exact numerical and approximate field-theoretic calculations. As a result, we establish the robust existence of a non-Gaussian universality class, and also construct the relevant low-energy effective field theory that could guide the understanding of quantum critical scaling for many strange metals. Using the kernel polynomial method (KPM), we provide numerical results for the calculated dynamical exponent ($z$) and correlation length exponent ($\nu$) for the disorder-driven semimetal (SM) to diffusive metal (DM) quantum phase transition at the Dirac point for several types of disorder, establishing its universal nature and obtaining the numerical scaling functions in agreement with our field-theoretical analysis.

III-Nitride quantum dots have emerged as a new chip-scale system for quantum information science, which combines electrical and optical interfaces on a semiconductor chip that is compatible with noncryogenic operating temperatures. Yet most work has been limited to optical excitations. To enable single-spin-based quantum optical and quantum information research, we demonstrate here quantized charging in optically active, site-controlled III-nitride quantum dots. Single-electron charging was confirmed by the voltage dependence of the energy, dipole moment, fine structures, and polarization properties of the exciton states in the quantum dots. The fundamental energy structures of the quantum dots were identified, including neutral and charged excitons, fine structures of excitons, and A and B excitons. The results lay the ground for coherent control of single charges in III-nitride quantum dots, opening a door to III-nitride-based spintronics and spin-qubit quantum information processing.

Using the tight-binding approach, we study the influence of a perpendicular magnetic field on the energy levels of hexagonal, triangular, and circular bilayer graphene (BLG) quantum dots (QDs) with zigzag and armchair edges. We obtain the energy levels for AB (Bernal)-stacked BLG QDs in both the absence and the presence of a perpendicular electric field (i.e., biased BLG QDs). We find different regions in the spectrum of biased QDs with respect to the crossing point between the lowest-electron and -hole Landau levels of a biased BLG sheet. Those different regions correspond to electron states that are localized at the center, edge, or corner of the BLG QD. Quantum Hall corner states are found to be absent in circular BLG QDs. The spatial symmetry of the carrier density distribution is related to the symmetry of the confinement potential, the position of zigzag edges, and the presence or absence of interlayer inversion symmetry.

We study the enhanced spin relaxation time of Au nanoparticles in nanopillar-shaped double-barrier junction devices with a stacked Fe/MgO/Au-nanoparticle/MgO/Fe structure. The size of Au nanoparticles located in a current path is deduced from a transmission electron micrograph and the Coulomb blockade behavior in the current-voltage characteristics of the devices. A finite tunnel magnetoresistance (TMR) is observed above a critical current and is attributable to spin accumulation in Au nanoparticles. Based on a simple model of TMR due to spin accumulation in a nanoparticle, the spin relaxation time ${\tau }_s$ is estimated from the magnitude of the critical current. The temperature and bias-voltage region where TMR appears are determined from systematic observations, showing that the appearance of TMR is not associated with the Coulomb blockade but with spin accumulation. We find that the obtained ${\tau }_s$ is anomalously extended (∼800 ns) at low temperatures and abruptly decreases above a critical temperature. Interestingly, the critical temperature strongly depends on the size of the Au nanoparticles and is much lower than the effective temperature corresponding to the discrete energy spacing. A theoretical analysis for the spin relaxation of electrons with discrete energy levels shows that not only the anomalously extended spin relaxation time, but also the strong temperature dependence of ${\tau }_s$ arise from the broadening of discrete energy levels due to coupling with phonons in the surrounding matrix. Numerical calculations using reasonable parameter values well reproduce the observed temperature and size dependence of the spin relaxation time in Au nanoparticles.

The magnetic phase diagram of a polar ferrimagnet ${\mathrm{CaBaFe}}_4{\mathrm{O}}_7$ with a magnetic easy axis has been investigated by measurements of magnetization, specific heat, and magnetoelectricity. A ferrimagnetic transition takes place at $T_{\mathrm{C}1}=275$ K within the orthorhombic phase followed by a second magnetic transition at $T_{\mathrm{C}2}=211$ K. Below $T_{\mathrm{C}2}$, successive metamagnetic transitions occur for magnetic fields applied perpendicular to the easy axis, implying a sequential emergence of magnetic states which are neither collinear nor coplanar. The observation of the static magnetoelectric effect was limited to temperatures below 120 K due to the conducting nature of the crystals at higher temperatures. The magnitude of the ferroelectric polarization shows large changes between the different field-induced magnetic phases. The low-field state is characterized by a large linear magnetoelectric coefficient of ${\alpha }_{cc}=39$ ps/m, while a gigantic polarization change of $\mathrm{\Delta }P=850 \mu \mathrm{C}/{\mathrm{m}}^2$ is observed for ${\mu }_{o}H=14$ T applied along the easy axis.

We investigate magnetoelectric couplings between an electric polarization and an antisymmetric spin pair, ${\mathbf{S}}_i\times {\mathbf{S}}_j$, in a multiorbital Hubbard model on a distorted lattice. We microscopically derive a generic form of the electric polarization, ${\mathbf{p}}_{\mathrm{AS}}=\widehat{d}({\mathbf{S}}_i\times {\mathbf{S}}_j)$, with a tensor, $\widehat{d}$, which includes the electric polarization induced by the Katsura-Nagaosa-Balatsky formula as a subset. The origin and nature of these magnetoelectric couplings are clarified in a unified way; the results indicate that various noncollinear magnetic structures, such as canted antiferromagnetic, proper screw, and ${120}^{\circ }$ spin structures, show multiferroic behaviors owing to non-Katsura-Nagaosa-Balatsky coupling.

Raman spectroscopy and powder x-ray diffraction methods have been used to characterize a novel phase of nitrogen which forms on compression from ambient pressure at low temperatures. The new, $\lambda$, phase exhibits an exceptionally wide range of pressure stability from below 1 to 140 GPa, overlapping nine other known phases. On heating, its transformations are different to those observed in other phases, implying that the phase nitrogen adopts depends not only on $P\text{−}T$ path, but also on the initial structural configuration, which greatly complicates its phase diagram.