We analyze the microscopic origin and the physical properties of the superconducting phase recently observed in MoS${}_2$. We show how the combination of the valley structure of the conduction band, the density dependence of the screening of the long-range Coulomb interactions, the short-range electronic repulsion, and the relative weakness of the electron-phonon interactions makes possible the existence of a phase where the superconducting order parameter has opposite signs in different valleys, resembling the superconductivity found in the pnictides and cuprates.

RbFe(MoO${}_4$)${}_2$ is a quasi-two-dimensional (quasi-2D) triangular lattice antiferromagnet (TLA) that displays a zero-field magnetically driven multiferroic phase with a chiral spin structure. By inelastic neutron scattering, we determine quantitatively the spin Hamiltonian. We show that the easy-plane anisotropy is nearly 1/3 of the dominant spin exchange, making RbFe(MoO${}_4$)${}_2$ an excellent system for studying the physics of the model 2D easy-plane TLA. Our measurements demonstrate magnetic-field-induced fluctuations in this material to stabilize the generic finite-field phases of the 2D $\mathit{XY}$ TLA. We further explain how Dzyaloshinskii-Moriya interactions can generate ferroelectricity only in the zero-field phase. Our conclusion is that multiferroicity in RbFe(MoO${}_4$)${}_2$, and its absence at high fields, results from the generic properties of the 2D $\mathit{XY}$ TLA.

We investigate the strain-induced coupling between a nitrogen-vacancy impurity and a resonant vibrational mode of a diamond nanoresonator. We show that under near-resonant laser excitation of the electronic states of the impurity, this coupling can modify the state of the resonator and either cool the resonator close to the vibrational ground state or drive it into a large-amplitude coherent state. We derive a semiclassical model to describe both effects and evaluate the stationary state of the resonator mode under various driving conditions. In particular, we find that by exploiting resonant single- and multiphonon transitions between near-degenerate electronic states, the coupling to high-frequency vibrational modes can be significantly enhanced and dominate over the intrinsic mechanical dissipation. Our results show that a single nitrogen-vacancy impurity can provide a versatile tool to manipulate and probe individual phonon modes in nanoscale diamond structures.

The stable organic radical 2-(4,5,6,7-tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (F4BImNN) forms hydrogen-bonded chains in the solid state and exhibits one-dimensional ferromagnetic exchange with $J/k_{\mathrm{B}}=22$ K. We use muon-spin rotation to demonstrate that weak interchain interactions drive the system to long-range magnetic order below $T_{\mathrm{c}}=0.72$ K. We use density-functional calculations of the muon site and compare our results to those obtained on the nonfluorinated analog compound. Our results show that F4BImNN is the best realization of a one-dimensional Heisenberg ferromagnet among purely organic compounds yet discovered.

A massless Dirac fermion ststem was realized in $\alpha$-(BEDT-TTF)${}_2$I${}_3$ [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene] under high pressure. In contrast to graphene, this is the first bulk (multilayered) massless Dirac fermion material. Another important difference from graphene is that this system has highly tilted Dirac cones. In this case, conventional chiral symmetry is broken under a magnetic field. Here we experimentally addressed the following question: Is the Landau level structure of the system with tilted Dirac cones the same as that of graphene [conventional two-dimensional (2D) Dirac fermion system] under a magnetic field? The answer is yes. We succeeded in injecting holes into $\alpha$-(BEDT-TTF)${}_2$I${}_3$ under high pressure. The detection of Shubnikov–de Haas oscillations whose phase was modified by Berry's phase $\pi$ is direct evidence that this system is truly a 2D Dirac fermion system. In addition, we revealed the energy diagram of this device and characterized the multilayered quantum Hall effect.

Monolayer fluctuations in the thickness of a semiconductor quantum well (QW) lead to the formation of spectrally resolved excitons located in the narrower, average, and thicker regions of the QW. Whether or not these excitons are coherently coupled via Coulomb interaction is a long-standing debate. We demonstrate that different types of disorder potential govern coherent coupling among excitons, and the coupling strength can be quantitatively measured using optical two-dimensional Fourier transform spectroscopy. Strong coherent coupling occurs between certain types of excitons but is missing between other types of excitons because the distinctive nature of excitons results in different spatial overlap. Our finding may be applicable to other disordered systems, such as photosynthesis and conjugated polymers, where exciton coupling plays a critical role in determining charge and energy transfer.

We simulate the electron transport across the Au(111)-pentacene interface using nonequilibrium Green's functions and density-functional theory (NEGF-DFT), and calculate the bias-dependent electron transmission. We find that the electrical contact resistance is dominated by the formation of a Schottky barrier at the interface, and show that the conventional semiconductor transport models across Schottky barriers need to be modified in order to describe the simulation data. We present an extension of the conventional Schottky barrier transport model, which can describe our simulation results and rationalize recent experimental data.

We report ultrafast pump-probe spectroscopy examining exciton dynamics in atomically thin MoS${}_2$. Spectrally and temporally resolved measurements are performed to investigate the interaction dynamics of two important direct-gap excitons ($A$ and $B$) and their associated broadening kinetics. The two excitons show strongly correlated interexcitonic dynamic, in which the transient blue-shifted excitonic absorption originates from the internal A-B excitonic interaction. The observed complex spectral response is determined by the exciton collision-induced linewidth broadening; the broadening of the $B$-exciton linewidth in turn lowers the peak spectral amplitude of the $A$ exciton. Resonant excitation at the $B$-exciton energy reveals that interexcitonic scattering plays a more important role in determining the broadening kinetics than free-carrier scattering.

Motivated by the recent experimental realization of a quantum interference transistor based on the superconducting proximity effect, we here demonstrate that the inclusion of a textured ferromagnet both strongly enhances the flux sensitivity of such a device and additionally allows for singlet-triplet switching by tuning a bias voltage. This functionality makes explicit use of the induced spin-triplet correlations due to the magnetic texture. Whereas the existence of such triplet correlations is well known, our finding demonstrates how spin-triplet superconductivity may be utilized for concrete technology, namely, to improve the functionality of ultrasensitive magnetometers.

Magnetic-field gradients are important for single-site addressability and electric-dipole spin resonance of spin qubits in semiconductor devices. We show that these advantages are offset by a potential reduction in coherence time due to the nonuniformity of the magnetic field experienced by a nuclear-spin bath interacting with the spin qubit. We theoretically study spins confined to quantum dots or at single donor impurities, considering both free-induction and spin-echo decay. For quantum dots in GaAs, we find that, in a realistic setting, a magnetic-field gradient can reduce the Hahn-echo coherence time by almost an order of magnitude. This problem can, however, be resolved by applying a moderate external magnetic field to enter a motional-averaging regime. For quantum dots in silicon, we predict a crossover from non-Markovian to Markovian behavior that is unique to these devices. Finally, for very small systems such as single phosphorus donors in silicon, we predict a breakdown of the common Gaussian approximation due to finite-size effects.

The magneto-optical longitudinal, transverse Hall, and circularly polarized responses of silicene and other materials described by a Kane-Mele Hamiltonian are calculated. Particular attention is paid to the effects of an external electric field and finite charge doping. The energy of interband transitions can be tuned by varying the electric field. The onset frequency of the absorptive peaks moves differently between the topological insulator and band insulator regimes. This may be used to verify experimentally the existence of the two insulating phases as well as provide a measure of the spin-orbit band gap. The zeroth Landau level splits between four spin and valley distinct energies resulting in valley-spin-polarized levels in the density of states. With charge doping, transitions between these levels allow for a spin- and valley-polarized response in the conductivity whereby charge carriers of specific spin and valley index can be isolated by tuning the incident photon frequency. Increasing the chemical potential is shown to redistribute spectral weight from interband transitions to a strong low-energy intraband response. For large chemical potential, this intraband feature is associated with the semiclassical cyclotron resonance frequency which is shown to linearly increase with magnetic field.

We present a detailed ${}^7$Li- and ${}^{31}$P-NMR study on single crystalline LiMnPO${}_4$ in the paramagnetic and antiferromagnetic phase (AFM, $T_N\sim 34$ K). This allows us to determine the spin directions in the field-induced spin-flop phase. In addition, the anisotropic dipolar hyperfine coupling tensor of the ${}^7$Li and ${}^{31}$P nuclei is also fully determined by orientation and temperature-dependent NMR experiments and compared to the calculated values from crystal structure data. Deviations of the experimental values from the theoretical ones are discussed in terms of Mn disorder which is induced by Li disorder. In fact, the disorder in the Mn sublattice is directly revealed by diffuse x-ray scattering data. The present results provide experimental evidence for the Li diffusion strongly coupling to structural distortions within the MnPO${}_4$ host, which is expected to significantly affect the Li mobility as well as the performance of batteries based on this material.

Quantum correlations quantified by quantum discord have been experimentally demonstrated as important physical resources in quantum information processing. Because of the inevitable interactions between the quantum system and the environment, it is essential to characterize the quantum correlation's dynamics in open systems and to protect the quantum correlation from being lost in the environment. Herein it is experimentally demonstrated in the Si:P system that there is a sudden change (at a critical time $t_c\approx 166$ ns) in the decay rates of classical and quantum correlations. To protect the quantum correlation, we achieve an extension of the critical time by 50 times to $8\mu$s by applying a two-flip dynamical decoupling pulse sequence. Moreover, we observe the phenomenon of the revival of both quantum and classical correlations. The experimental observation of a nondecay interval for quantum correlation and its great extension in a solid-state system with genuine noise makes the use of quantum discord as a physical resource in quantum information processing more practicable.

The magnetic properties of Mn, Co, and Ni substituted Fe${}_4$N are calculated from first principles theory. It is found that the generalized gradient approximation reproduces with good accuracy the magnetic moment and equilibrium volume for the parent Fe${}_4$N structure, with the atomic moment largest for the Fe atom furthest away from the N atom (Fe I site), approaching a value of 3 ${\mu }_B$/atom, whereas the Fe atom closer to the N atom (Fe II site) has a moment closer to that of bcc Fe. The substitution of Fe for Mn, Co, or Ni, shows an intricate behavior in which the Mn substitution clearly favors the Fe II site, Ni favors substitution on the Fe I site, and Co shows no strong preference for either lattice site. The Ni and Co substitution results in a ferromagnetic coupling to the Fe atoms, whereas Mn couples antiferromagnetically on the Fe II site and ferromagnetically on the Fe I site. For all types of doping, the total magnetic moment is enhanced compared to Fe${}_4$N only in the energetically very unfavorable case of Mn doping at the Fe I site.

The effects of Gd doping and O vacancies on the magnetic interaction and Curie temperature $T_C$ of EuO are studied using first-principles calculations. Linear response calculations in the virtual crystal approximation show a broad maximum in the Curie temperature as a function of doping, which results from the combination of the saturating contribution from indirect exchange and a decreasing contribution from the $f$-$d$ hopping mechanism. Non-Heisenberg interaction at low doping levels and its effect on $T_C$ are examined. The electronic structure of a substitutional Gd and of an O vacancy in EuO are evaluated. When the $4f$ spins are disordered, the impurity state goes from single to double occupation, but correlated bound magnetic polarons are not ruled out. At higher vacancy concentrations typical for Eu-rich EuO films, the impurity states broaden into bands and remain partially filled. To go beyond the homogeneous doping picture, magnetostructural cluster expansions are constructed, which describe the modified exchange parameters near Gd dopants or O vacancies. Thermodynamic properties are studied using Monte Carlo simulations. The Curie temperature for Gd-doped EuO agrees with the results of the virtual crystal approximation and shows a maximum of about 150 K. At 3.125% vacancy concentration the $T_C$ increases to 120 K, consistent with experimental data for Eu-rich film samples.

The disordered quasi-one-dimensional magnet BaCu${}_2$SiGeO${}_7$ is considered as one of the best physical realizations of the random Heisenberg chain model, which features an irregular distribution of the exchange parameters and whose ground state is predicted to be the scarcely investigated random-singlet state (RSS). Based on extensive ${}^{29}$Si NMR and magnetization studies of BaCu${}_2$SiGeO${}_7$, combined with numerical quantum Monte Carlo simulations, we obtain remarkable quantitative agreement with theoretical predictions of the random Heisenberg chain model and strong indications for the formation of a random-singlet state at low temperatures in this compound. As a local probe, NMR is a well-adapted technique for studying the magnetism of disordered systems. In this case, it also reveals an additional local transverse staggered field, which affects the low-temperature properties of the RSS. The proposed model Hamiltonian satisfactorily accounts for the temperature dependence of the NMR line shapes.

The magnetic behavior of ultrathin ferromagnetic films deposited on substrates is strongly affected by the properties of the substrate. We investigate the spin pumping rate, interlayer exchange coupling, and dynamic exchange coupling between ultrathin ferromagnetic films through palladium, a nonmagnetic substrate that displays strong Stoner enhancement. We find that the interlayer exchange coupling, both in the static and dynamic versions, is qualitatively affected by the electron-electron interaction within the substrate. For instance, the oscillatory behavior that is a hallmark property of the RKKY exchange coupling is strongly suppressed by the induced moment in Pd. Although the spin pumping rate of ferromagnetic films atop palladium is only mildly changed by the electron-electron interaction in Pd, the change is large enough to be detected experimentally. The qualitative aspects of our results for palladium are expected to remain valid for any nonmagnetic substrate where Coulomb repulsion is large.

Transmission x-ray diffraction experiments have been carried out on solid helium grown in porous Vycor glass. Measurements were made at temperatures near 0.7 K and at pressures up to 162 bars. The crystalline phases of solid helium in Vycor are found to differ significantly from the bulk. At pressures from 70 bars through 98 bars the helium is polycrystalline and displays a single size broadened scattering peak. Above 98 bars the peak splits into three close peaks. No higher order peaks are seen at any pressure, indicating significant reduction in intensity due to disorder. A broad peak is present at all pressures, which may indicate the presence of amorphous solid. We tentatively identify the low-pressure phase as bcc and the high-pressure phase as coexistence between bcc and hcp. Size broadening indicates an average grain size of approximately 5 nm, comparable to the Vycor pore size.

We have simulated the thermodynamics of vortices in a thin film of a type-II superconductor. We make the lowest Landau level approximation, and use quasiperiodic boundary conditions. Our work is consistent with the results of previous simulations where evidence was found for an apparent first-order transition between the vortex liquid and the vortex crystal states. We show, however, that these results are just an artifact of studying systems that are too small. There are substantial difficulties in simulating larger systems using traditional approaches. By means of the optimal energy diffusion algorithm, we have been able to study systems containing up to about one thousand vortices, and for these larger systems, the evidence for a first-order transition disappears. By studying both crystalline and hexatic orders, we show that the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) scenario seems to apply where melting from the crystal is first to the hexatic liquid state and next to the normal vortex liquid, in both cases via a continuous transition.