The low-temperature magnetic behavior of the double-layered ruthenate Sr${}_3$Ru${}_2$O${}_7$, as grown from a eutectic Sr${}_2$RuO${}_4$-Sr${}_3$Ru${}_2$O${}_7$ system, was investigated via zero- and transverse-field muon-spin rotation. The gradual increase of the muon relaxation rate observed below 2.5 K, even in the absence of applied magnetic fields, indicates the occurrence of a spontaneous breaking of time-reversal symmetry. The onset of the latter at a temperature above 1.5 K, the $T_c$ of the single phase Sr${}_2$RuO${}_4$, provides evidence about an unconventional superconducting state in the eutectic phase, which most likely takes place at the interface between the Sr${}_2$RuO${}_4$ and Sr${}_3$Ru${}_2$O${}_7$ domains, or even inside the Sr${}_3$Ru${}_2$O${}_7$ phase. We show that the superconducting state manifests a two-component behavior in the transverse-field response with change-over at about $T=2.5$ K and $T=1.5$ K. The comparison with zero-field $\mu$SR data in the Ru-Sr${}_2$RuO${}_4$ eutectic system rules out the possibility of spurious effects due to embedded Ru islands.

We investigate the magnetic polarization of the Ir $5d$ dopant states in the pnictide superconductor Ba(Fe${}_{1-x}$Ir${}_x$)${}_2$As${}_2$ with $x=0.027\left(2\right)$ using Ir $L_3$ edge x-ray resonant magnetic scattering (XRMS). Despite the fact that doping partially suppresses the antiferromagnetic transition, we find that magnetic order survives around the Ir dopant sites. The Ir states are magnetically polarized with commensurate stripe-like antiferromagnetic order and long correlations lengths, ${\xi }_{\mathrm{mag}}>2800$ and $>$850 Å, in the $ab$ plane and along the $c$ axis, respectively, driven by their interaction with the Fe spins. This Ir magnetic order persists up to the Néel transition of the majority Fe spins at $T_N=74\left(2\right)$ K. At 5 K we find that magnetic order coexists microscopically with superconductivity in Ba(Fe${}_{1-x}$Ir${}_x$)${}_2$As${}_2$. The energy dependence of the XRMS through the Ir $L_3$ edge shows a non-Lorentzian line shape, which we explain in terms of interference between Ir resonant scattering and Fe nonresonant magnetic scattering.

Highly $s{p}^3$-bonded, nearly hydrogen-free carbon-based materials can exhibit extremely low friction and wear in the absence of any liquid lubricant, but this physical behavior is limited by the vapor environment. The effect of water vapor on friction and wear is examined as a function of applied normal force for two such materials in thin film form: one that is fully amorphous in structure (tetrahedral amorphous carbon, or ta-C) and one that is polycrystalline with $<10$ nm grains [ultrananocrystalline diamond (UNCD)]. Tribologically induced changes in the chemistry and carbon bond hybridization at the surface are correlated with the effect of the sliding environment and loading conditions through ex situ, spatially resolved near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. At sufficiently high relative humidity (RH) levels and/or sufficiently low loads, both films quickly achieve a low steady-state friction coefficient and subsequently exhibit low wear. For both films, the number of cycles necessary to reach the steady-state is progressively reduced for increasing RH levels. Worn regions formed at lower RH and higher loads have a higher concentration of chemisorbed oxygen than those formed at higher RH, with the oxygen singly bonded as hydroxyl groups (C-OH). While some carbon rehybridization from $s{p}^3$ to disordered $s{p}^2$ bonding is observed, no crystalline graphite formation is observed for either film. Rather, the primary solid-lubrication mechanism is the passivation of dangling bonds by OH and H from the dissociation of vapor-phase H${}_2$O. This vapor-phase lubrication mechanism is highly effective, producing friction coefficients as low as 0.078 for ta-C and 0.008 for UNCD, and wear rates requiring thousands of sliding passes to produce a few nanometers of wear.

We report a systematic study of the hyperfine interaction between the electron spin of a single nitrogen-vacancy (NV) defect in diamond and nearby ${}^{13}\text{C}$ nuclear spins, by using pulsed electron-spin resonance spectroscopy. We isolate a set of discrete values of the hyperfine coupling strength ranging from 14 MHz to 400 kHz and corresponding to ${}^{13}\text{C}$ nuclear spins placed at different lattice sites of the diamond matrix. For each lattice site, the hyperfine interaction is further investigated through nuclear-spin polarization measurements and by studying the magnetic field dependence of the hyperfine splitting. This work provides information that is relevant for the development of nuclear-spin-based quantum register in diamond.

Magnetic properties of YbAl${}_3$C${}_3$ with the hexagonal ScAl${}_3$C${}_3$-type structure have been investigated by the magnetization ($M$) and specific-heat ($C$) measurements under magnetic fields ($H$). YbAl${}_3$C${}_3$ is reported to show a spin-gap state, which is considered to be ascribed to a magnetic dimer formed in the orthorhombic phase below the structural transition temperature ($T_{\mathrm{s}}=$ 77 K). Present study has revealed history-dependent magnetic properties below $T_{\mathrm{s}}$ and field-induced anomalous magnetic states at low temperatures. The former is considered to be attributed to the cross correlation between the structural deformation and the magnetic field similar to those observed in multiferroic materials, although YbAl${}_3$C${}_3$ below $T_{\mathrm{s}}$ is not in the ferromagnetic state but certainly in the dimer state. The latter is partially similar to that observed in the field-induced ordered phase (FIOP) of $d$-electron dimer systems. The $M$ versus $H$ curve at low temperatures exhibits a kink at a certain magnetic field and then it increases in proportion to the field like that of FIOP. However, neither kink nor peak suggesting the emergence of FIOP is observed in the temperature dependence of both $M$($T$) and $C$($T$). Instead, $C/T$ shows an anomalous increase proportional to $-$ln$T$ with decreasing temperature in a finite field range, suggesting an anomalous disordered state such as a non-Fermi-liquid state in a strongly correlated $f$-electron system. These anomalous magnetic states may be partially relevant to characteristics of $f$-electron dimer system.

The spin interaction between an electron and nuclei was investigated optically in a single self-assembled InAlAs quantum dot (QD). In spin dynamics at the initial stage, the fluctuation of nuclear field and the resulting electron spin relaxation time play a crucial role. We examined a positively charged exciton in a QD to evaluate the key physical quantities directly via the temporal evolution measurements of the Overhauser shift and the degree of circular polarization. In addition, the validity of our used spin dynamics model was discussed in the context of the experimentally obtained key parameters.

The general Lagrange-Euler formalism for the three memory circuit elements, namely, memristive, memcapacitive, and meminductive systems, is introduced. In addition, mutual meminductance, i.e., mutual inductance with a state depending on the past evolution of the system, is defined. The Lagrange-Euler formalism for a general circuit network, the related work-energy theorem, and the generalized Joule's first law are also obtained. Examples of this formalism applied to specific circuits are provided, and the corresponding Hamiltonian and its quantization for the case of nondissipative elements are discussed. The notion of memory quanta, the quantum excitations of the memory degrees of freedom, is presented. Specific examples are used to show that the coupling between these quanta and the well-known charge quanta can lead to a splitting of degenerate levels and to other experimentally observable quantum effects.

Periodically driven systems, which can be described by Floquet theory, have been proposed to show characteristic behavior that is distinct from static Hamiltonians. Floquet theory proposes to describe such periodically driven systems in terms of states that are indexed by a photon number in addition to the usual Hilbert space of the system. We propose a way to measure directly this additional Floquet degree of freedom by the measurement of the DC conductance of a single channel quantum point contact. Specifically, we show that a single channel wire augmented with a grating structure when irradiated with microwave radiation can show a DC conductance above the limit of one conductance quantum set by the Landauer formula. Another interesting feature of the proposed system is that being nonadiabatic in character, it can be used to pump a strong gate-voltage-dependent photocurrent even with linearly polarized radiation.

We use self-consistent quantum transport theory to investigate the influence of electron-electron interactions on interlayer transport in semiconductor electron bilayers in the absence of an external magnetic field. We conclude that, even though spontaneous pseudospin order does not occur at zero field, interaction-enhanced quasiparticle tunneling amplitudes and pseudospin transfer torques do alter tunneling I-V characteristics, and can lead to time-dependent response to a dc bias voltage.

The coupling of angstrom-sized electron probes with spin-polarized electronic transitions shows that the inelastically scattered probe electron is in a mixed state containing electron vortices with nonzero orbital angular momentum. These electrons create an asymmetric intensity distribution in energy filtered diffraction patterns, giving access to maps of the magnetic moments with atomic resolution. A feasibility experiment shows evidence of the predicted effect. Potential applications are column-by-column maps of magnetic ordering, and the creation of angstrom-sized free electrons with orbital angular momentum by inelastic scattering in a thin ferromagnetic foil.

A time-reversal invariant Kitaev-type model is introduced in which spins (Dirac matrices) on the square lattice interact via anisotropic nearest-neighbor and next-nearest-neighbor exchange interactions. The model is exactly solved by mapping it onto a tight-binding model of free Majorana fermions coupled with static ${\mathbb{Z}}_2$ gauge fields. The Majorana fermion model can be viewed as a model of time-reversal-invariant superconductor and is classified as a member of symmetry class DIII in the Altland-Zirnbauer classification. The ground-state phase diagram has two topologically distinct gapped phases which are distinguished by a ${\mathbb{Z}}_2$ topological invariant. The topologically nontrivial phase supports both a Kramers’ pair of gapless Majorana edge modes at the boundary and a Kramers’ pair of zero-energy Majorana states bound to a 0-flux vortex in the $\pi$-flux background. Power-law decaying correlation functions of spins along the edge are obtained by taking the gapless Majorana edge modes into account. The model is also defined on the one-dimension ladder, in which case again the ground-state phase diagram has ${\mathbb{Z}}_2$ trivial and nontrivial phases.

The preparation of a coherent heavy-hole spin via ionization of a spin-polarized electron-hole pair in an InAs/GaAs quantum dot in a Voigt geometry magnetic field is experimentally investigated. For a dot with a typical bright-exciton fine-structure splitting of $17\mu \mathrm{eV}$, the fidelity of the spin preparation is limited to 0.75, with optimum preparation occurring when the effective fine structure of the bright exciton matches the in-plane hole Zeeman energy. In principle, higher fidelities can be achieved by minimizing the bright-exciton fine-structure splitting.

We propose to measure the spectrum of magnetic excitation in magnetic materials using the motion of a vortex lattice driven by both ac and dc currents in superconductors. When the motion of a vortex lattice is resonant with the oscillation of magnetic moments, the voltage decreases at a given current. From transport measurement, one can obtain the frequency of the magnetic excitation with the wave number determined by the vortex-lattice constant. By changing the lattice constant through applied magnetic fields, one can obtain the spectrum of the magnetic excitation up to a wave vector of order $10{\mathrm{nm}}^{-1}$.

We study a lattice model of interacting loops in three dimensions with a $1/r^2$ interaction. Using Monte Carlo methods, we have found that the phase diagram contains a line of second-order phase transitions between a phase where the loops are gapped and a phase where they proliferate. The correlation length exponent and critical conductivity vary continuously along this line. Our model is exactly self-dual at a special point on the critical line, which allows us to calculate the critical conductivity exactly at this point.

We demonstrate how ab initio cluster calculations including the full Coulomb vertex can be done in the basis of the localized Wannier orbitals which describe the low-energy density functional (local-density approximation) band structure of an infinite crystal, e.g., the transition-metal $3d$ and oxygen $2p$ orbitals. The spatial extent of our $3d$ Wannier orbitals (orthonormalized $N$th-order muffin-tin orbitals) is close to that found for atomic Hartree-Fock orbitals. We define ligand orbitals as those linear combinations of the O $2p$ Wannier orbitals which couple to the $3d$ orbitals for the chosen cluster. The use of ligand orbitals allows for a minimal Hilbert space in multiplet ligand-field theory calculations, thus reducing the computational costs substantially. The result is a fast and simple ab initio theory, which can provide useful information about local properties of correlated insulators. We compare results for NiO, MnO, and SrTiO${}_3$ with x-ray absorption, inelastic x-ray scattering, and photoemission experiments. The multiplet ligand-field theory parameters found by our ab initio method agree within $\sim 10\%$ with known experimental values.

A strong electron-phonon interaction which limits the electronic mobility of semiconductors can also have significant effects on phonon frequencies. The latter is the key to the use of Raman spectroscopy for nondestructive characterization of doping in graphene-based devices. Using in situ Raman scattering from a single-layer MoS${}_2$ electrochemically top-gated field-effect transistor (FET), we show softening and broadening of the $A_{1g}$ phonon with electron doping, whereas the other Raman-active $E_{2g}^1$ mode remains essentially inert. Confirming these results with first-principles density functional theory based calculations, we use group theoretical arguments to explain why the $A_{1g}$ mode specifically exhibits a strong sensitivity to electron doping. Our work opens up the use of Raman spectroscopy in probing the level of doping in single-layer MoS${}_2$-based FETs, which have a high on-off ratio and are of technological significance.

We report the energy level structure of the ${}^3$H${}_6$ and ${}^3$H${}_4$ multiplets for Tm${}^{3+}$ doped congruent LiNbO${}_3$, as well as the decoherence properties and their temperature dependencies for the ${}^3$H${}_6\left(1\right){\leftrightarrow }^3$H${}_4\left(1a\right)$ transition at 794 nm. It is shown that this material provides very significant improvements in bandwidth, time-bandwidth product, and sensitivity for spatial-spectral holographic signal processing devices and quantum memories based on spectral hole burning. The available signal processing bandwidth for 0.1$\%$Tm${}^{3+}$:LiNbO${}_3$ is 300 GHz versus 20 GHz for Tm${}^{3+}$YAG. The peak absorption coefficient for 0.1$\%$Tm${}^{3+}$:LiNbO${}_3$ is 15 cm${}^{-1}$ at 794.5 nm compared with 1.7 cm${}^{-1}$ for 0.1$\%$Tm:YAG at 793 nm, and the total absorption strength is eighty times stronger. The oscillator strength for Tm${}^{3+}$:LiNbO${}_3$ is about twenty-five times larger than that for Tm${}^{3+}$:YAG, making the material five times more sensitive for processing high-bandwidth analog signals. The homogeneous linewidth, which determines processing time or spectrum analyzer resolution, is 30 kHz at 1.6 K and 350 kHz at 6 K, as measured by photon echoes. Those values establish potential time-bandwidth products of 10${}^7$ and 7 $\times$10${}^5$, respectively. The temperature dependence of the homogeneous linewidth was explained by observation of a 7.8 cm${}^{-1}$ crystal field level in the ground multiplet and direct phonon coupling. The excited state ${}^3$H${}_4$ lifetime $T_1$ is 152 $\mu$s and the bottleneck lifetime of the lowest ${}^3$F${}_4$ level is 7 ms from photon echo measurements. These factors combine to provide a surprisingly large increase in key parameters that determine material performance for spatial-spectral holography, quantum information, and other spectral hole burning applications.

We operate a superconducting quantum processor consisting of two tunable transmon qubits coupled by a swapping interaction, and equipped with nondestructive single-shot readout of the two qubits. With this processor, we run the Grover search algorithm among four objects and find that the correct answer is retrieved after a single run with a success probability between $0.52$ and $0.67$, which is significantly larger than the $0.25$ achieved with a classical algorithm. This constitutes a proof of concept for the quantum speed-up of electrical quantum processors.

We demonstrate nonadiabatic charge pumping utilizing a sequence of coherent oscillations between a superconducting island and two reservoirs. The pumping rate for each elementary cycle is limited by the coupling between the island and the reservoirs given by the Josephson energy. Our experimental and theoretical studies show that relaxation can be employed to reset the pump in order to avoid accumulation of errors due to nonideal control pulses. Thus our results demonstrate the effects of nonadiabatic quantum pumping and dissipation.

We have investigated the thermal Hall effect of magnons for various ferromagnetic insulators. For pyrochlore ferromagnetic insulators Lu${}_2$V${}_2$O${}_7$, Ho${}_2$V${}_2$O${}_7$, and In${}_2$Mn${}_2$O${}_7$, finite thermal Hall conductivities have been observed below the Curie temperature $T_C$. From the temperature and magnetic-field dependencies, it is concluded that magnons are responsible for the thermal Hall effect. The Hall effect of magnons can be well explained by the theory based on the Berry curvature in momentum space induced by the Dzyaloshinskii-Moriya (DM) interaction. The analysis has been extended to the transition-metal (TM) oxides with perovskite structure. The thermal Hall signal was absent or far smaller in La${}_2$NiMnO${}_6$ and YTiO${}_3$, which have the distorted perovskite structure with four TM ions in the unit cell. On the other hand, a finite thermal Hall response is discernible below $T_C$ in another ferromagentic perovskite oxide BiMnO${}_3$, which shows orbital ordering with a larger unit cell. The presence or absence of the thermal Hall effect in insulating pyrochlore and perovskite systems reflect the geometric and topological aspect of DM-induced magnon Hall effect.