Spin-noise measurements can serve as a direct probe for the microscopic decoherence mechanism of an electronic spin in semiconductor quantum dots (QDs). We have calculated the spin-noise spectrum in the anisotropic central spin model using a Chebyshev expansion technique which exactly accounts for the dynamics up to an arbitrary long but fixed time in a finite-size system. In the isotropic case, describing QD charge with a single electron, the short-time dynamics is in good agreement with quasistatic approximations for the thermodynamic limit. The spin-noise spectrum, however, shows strong deviations at low frequencies with a power-law behavior of ${\omega }^{-3/4}$ corresponding to a $t^{-1/4}$ decay at intermediate and long times. In the Ising limit, applicable to QDs with heavy-hole spins, the spin-noise spectrum exhibits a threshold behavior of ${(\omega -{\omega }_L)}^{-1/2}$ above the Larmor frequency ${\omega }_L=g{\mu }_{B}B$. In the generic anisotropic central spin model we have found a crossover from a Gaussian type of spin-noise spectrum to a more Ising-type spectrum with increasing anisotropy in a finite magnetic field. In order to make contact with experiments, we present ensemble averaged spin-noise spectra for QD ensembles charged with single electrons or holes. The Gaussian-type noise spectrum evolves to a more Lorentzian shape spectrum with increasing spread of characteristic time scales and $g$ factors of the individual QDs.

Nitrogen-vacancy centers in diamond are ideal platforms for quantum simulation, which allows one to handle problems that are intractable theoretically or experimentally. Here we propose a digital quantum simulation scheme to simulate the quantum phase transition occurring in an ultrathin topological insulator film placed in a parallel magnetic field [Zyuzin et al., Phys. Rev. B 83, 245428 (2011)]. The quantum simulator employs high quality spin qubits achievable in nitrogen-vacancy centers. With a series of precise controls, the dynamics of the spin system is made capable of simulating the dynamics of the simulated TI system. The eigenenergies of the TI system are extracted by a quantum algorithm of finding eigenvalues. The scheme can be realized with existing technologies. An operation time of the order of 100 $\mu$s is estimated for each individual run.

We have measured the drag force from quantum turbulence on a series of quartz tuning forks in superfluid helium. The tuning forks were custom made from a 75-$\mu$m-thick wafer. They have identical prong widths and prong spacings, but different lengths to give different resonant frequencies. We have used both the fundamental and overtone flexure modes to probe the turbulent drag over a broad range of frequencies $f=\omega /2\pi$ from 6.5 to 300 kHz. Optical measurements show that the velocity profiles of the flexure modes are well described by a cantilever beam model. The critical velocity for the onset of quantum turbulence at low temperatures is measured to be $v_c\approx \sqrt{0.7\kappa \omega }$ where $\kappa$ is the circulation quantum. The drag from quantum turbulence shows a small frequency dependence when plotted against the scaled velocity $v/v_c$.

The motion of domain walls is influenced by their internal structure and their structural changes during motion. The macroscopic motion is well understood but the internal dynamics during motion have not been experimentally studied in detail. We study vortex domain walls excited by nanosecond long magnetic field pulses in a pinning potential and above the pinning threshold. The dynamic response is imaged by transmission x-ray microscopy and the structural changes are analyzed. From the directly observed inertial behavior of the wall the domain-wall mass equivalent is estimated. During motion, oscillations of both the vortex core around the center of the domain wall and the domain-wall width are observed. The wall shows a fully elastic behavior by compression and overexpansion when being pushed by the field.

The local spin susceptibility in superconducting LiFeAs was studied by polarized neutron diffraction as a function of temperature. In the superconducting phase the spin susceptibility is clearly suppressed and it can be well described by the Yosida function suggesting a singlet pairing to occur at low temperature. The spin susceptibility in the normal state and its suppression in the superconducting phase are fully comparable to observations in Co-doped BaFe${}_2$As${}_2$.

Recent muon spin-rotation ($\mu$SR) measurements on SrPtAs revealed time-reversal-symmetry breaking with the onset of superconductivity [Biswas et al., Phys. Rev. B 87, 180503(R) (2013)], suggesting an unconventional superconducting state. We investigate this possibility via the functional renormalization group and find a chiral $(d+id)$-wave order parameter favored by the multiband fermiology and hexagonal symmetry of SrPtAs. This $(d+id)$-wave state exhibits significant gap anisotropies as well as gap differences on the different bands, but only has point nodes on one of the bands at the Brillouin zone corners. We study the topological characteristics of this superconducting phase, which features Majorana-Weyl nodes in the bulk, protected surface states, and an associated thermal Hall response. The lack of extended nodes and the spontaneously broken time-reversal symmetry of the $(d+id)$-wave state are in agreement with the $\mu$SR experiments. Our theoretical findings, together with the experimental evidence, thus suggest that SrPtAs is an example of chiral $d$-wave pairing and a Weyl superconductor.

We study deterministic magnetic reversal of a perpendicularly magnetized Co layer in a Co/MgO/Ta nanosquare driven by spin Hall torque from an in-plane current flowing in an underlying Pt layer. The rate-limiting step of the switching process is domain wall (DW) depinning by spin Hall torque via a thermally assisted mechanism that eventually produces full reversal by domain expansion. An in-plane applied magnetic field collinear with the current is required, with the necessary field scale set by the need to overcome DW chirality imposed by the Dzyaloshinskii-Moriya interaction. Once Joule heating is taken into account the switching current density is quantitatively consistent with a spin Hall angle ${\theta }_{SH}\approx 0.07$ for 4 nm of Pt.

Regarding light absorption/emission efficiency, silicon presents the fundamental drawback of its indirect band gap. It is long known, though, that optical properties are greatly enhanced in materials which comprise different kinds of nanocrystalline Si covered by or embedded in Si oxide layers. Conversely, their amorphous counterparts have received far less attention, such that no general consensus about the emission mechanism prevails. We report here on an efficiently luminescent material based on amorphous Si nanoparticles (a-Si NPs) embedded in a nonstoichiometric Si oxide matrix, which exhibits intense, broadband emission from the a-Si NPs, spectrally separable from the defect luminescence of the suboxide matrix. Apart from the brightness of the emitted light, the nanometer-size a-Si inclusions present the technological advantage of needing very moderate annealing temperatures (450${}^{\circ }$C–700${}^{\circ }$C) for their production. The combined use of high pressure, experimentally as well as theoretically, allowed us to trace back the microscopic origin of the photoluminescence to radiative recombination processes between confined states of the a-Si NPs. The signature of quantum confinement is found in the magnitude and sign of the pressure coefficient of different optical transition energies. The pressure derivatives exhibit a universal dependence on particle size, determined solely by the confinement energy of the discrete electron state involved in the radiative recombination process.

Inhomogeneities and junctions in wires are natural sources of scattering, and hence resistance. A conducting fixed point usually requires an adiabatically smooth system. One notable exception is “healing,” which has been predicted in systems with special symmetries, where the system is driven to the homogeneous fixed point. Here we present theoretical results for a different type of conducting fixed point which occurs in inhomogeneous wires with an abrupt jump in hopping and interaction strength. We show that it is always possible to tune the system to an unstable conducting fixed point which does not correspond to translational invariance. We analyze the temperature scaling of correlation functions at and near this fixed point and show that two distinct boundary exponents appear, which correspond to different effective Luttinger liquid parameters. Even though the system consists of two separate interacting parts, the fixed point is described by a single conformally invariant boundary theory. We present details of the general effective bosonic field theory including the mode expansion and the finite size spectrum. The results are confirmed by numerical quantum Monte Carlo simulations on spinless fermions. We predict characteristic experimental signatures of the local density of states near junctions.

A computation of the cuprate phase diagram is presented. Adiabatic deformability back to the density function band structure plus symmetry constraints lead to a Fermi liquid theory with five interaction parameters. Two of these are forced to zero by experiment. The remaining three are fit to the moment of the antiferromagnetic state at half-filling, the superconducting gap at optimal doping, and the maximum pseudogap. The latter is identified as orbital antiferromagnetism. Solution of the Hartree-Fock equations gives, in quantitative agreement with experiment, (1) quantum phase transitions at 5% and 16% $p$-type doping, (2) insulation below 5%, (3) a $d$-wave pseudogap quasiparticle spectrum, (4) pseudogap and superconducting gap values as a function of doping, (5) superconducting $T_c$ versus doping, (6) London penetration depth versus doping, and (7) spin wave velocity. The fit points to superexchange mediated by the bonding O atom in the Cu-O plane as the causative agent of all three ordering phenomena.

We investigate the emission properties of a single semiconductor quantum dot deterministically coupled to a confined optical mode in the weak-coupling regime. A strong spectral pulling, broadening, and narrowing of the cavity mode emission is evidenced when changing the spectral detuning between the emitter and the cavity. These features are theoretically accounted for by considering the phonon assisted emission of the quantum dot transition. These observations highlight a regime of cavity quantum electrodynamics scarcely investigated so far, involving spectrally broad emitters.

We calculate the tunneling current through a Fabry-Pérot interferometer in the fractional quantum Hall regime. Within linear response theory (weak tunneling but arbitrary source-drain voltage), we find a general expression for the current due to tunneling of quasiparticles in terms of Carlson's $R$ function. Our result is valid for fractional quantum Hall states with an edge theory consisting of a charged channel and any number of neutral channels, with possibly different edge velocities and different chiralities. We analyze the case with a single neutral channel in detail, which applies for instance to the edge of the Moore-Read state. In addition, we consider an asymmetric interferometer with different edge lengths between the point contacts on opposite edges, and we study the behavior of the current as a function of varying edge length. Recent experiments attempted to measure the Aharanov-Bohm effect by changing the area inside the interferometer using a plunger gate. Theoretical analyses of these experiments have so far not taken into account the accompanying change in the edge lengths. We show that the tunneling current exhibits multiple oscillations as a function of this edge length, with frequencies proportional to the injected edge current and inversely proportional to the edge velocities. In particular, the edge velocities can be measured by looking at the Fourier spectrum of the edge current. We provide a numerical scheme to calculate and plot the $R$ function, and include sample plots for a variety of edge states with parameter values, which are experimentally relevant.

Anderson's idea of a (short-ranged) resonating valence bond (RVB) spin liquid has been the first ever proposal of what we now call a topologically ordered phase. Since then, a wealth of exactly solvable lattice models have been constructed that have topologically ordered ground states. For a long time, however, it has been difficult to realize Anderson's original vision in such solvable models, according to which the ground state has an unbroken SU(2) spin rotational symmetry and is dominated by fluctuation of singlet bonds. The kagome lattice is the simplest lattice geometry for which parent Hamiltonians stabilizing a prototypical spin-1$/$2 short-ranged RVB wave function has been constructed and strong evidence has been given that this state belongs to a topological phase. The uniqueness of the desired RVB-type ground states has, however, not been rigorously proven for the simplest possible such Hamiltonian, which acts on 12 spins at a time. Rather, this uniqueness has been demonstrated for a longer ranged (19-site) variant of this Hamiltonian by Schuch et al., via making contact with powerful results for projected entangled-pair states. In this paper, we extend this result to the 12-site Hamiltonian. Our result is based on numerical studies on finite clusters, for which we demonstrate a “ground state intersection property” with implications for arbitrary system size. We also review the relations between various constructions schemes for RVB parent Hamiltonians found in the literature.

A density functional theory (DFT) study of the 1/2$\langle 111\rangle$ screw dislocation was performed in the following body-centered cubic transition metals: V, Nb, Ta, Cr, Mo, W, and Fe. The energies of the easy, hard, and split core configurations, as well as the pathways between them, were investigated and used to generate the two-dimensional (2D) Peierls potential, i.e. the energy landscape seen by the dislocation as a function of its position in the (111) plane. In all investigated elements, the nondegenerate easy core is the minimum energy configuration, while the split core configuration, centered in the immediate vicinity of a $\langle 111\rangle$ atomic column, has a high energy near or above that of the hard core. This unexpected result yields 2D Peierls potentials very different from the usually assumed landscapes. The 2D Peierls potential in Fe differs from the other transition metals, with a monkey saddle instead of a local maximum located at the hard core. An estimation of the Peierls stress from the shape of the Peierls barrier is presented in all investigated metals. A strong group dependence of the core energy is also evidenced, related to the position of the Fermi level with respect to the minimum of the pseudogap of the electronic density of states.

We derive and discuss general physical bounds on the electromagnetic scattering and absorption of passive structures. Our theory, based on passivity and power conservation, quantifies the minimum and maximum allowed scattering for an object that absorbs a given level of power. We show that there is a fundamental tradeoff between absorption and overall scattering suppression for each scattering harmonic, providing a tool to quantify the performance of furtive sensors, regardless of the applied principle for scattering suppression. We illustrate these fundamental limitations with examples of light scattering from absorbing plasmonic nanoparticles and loaded dipole antennas, envisioning applications to the design of cloaked sensors and absorbers with maximized absorption efficiency.

We investigate the properties of a hybrid single-electron transistor, involving a small superconducting island sandwiched between normal metal leads, which is driven by dc plus ac voltages. In order to describe its properties we derive from the microscopic theory a set of coupled equations. They consist of a master equation for the probability to find excess charges on the island, with rates depending on the distribution of nonequilibrium quasiparticles. Their dynamics follows from a kinetic equation which accounts for the excitation by single-electron tunneling as well as the relaxation and eventual recombination due to the interaction with phonons. Our low-temperature results compare well with recent experimental findings obtained for ac-driven hybrid single-electron turnstiles.

The carbon isotope ${}^{13}$C, in contrast to ${}^{12}$C, possesses a nuclear magnetic moment and can induce electron spin dephasing in graphene. This effect is usually neglected due to the low abundance of ${}^{13}$C in natural carbon allotropes ($\sim$1%). Chemical vapor deposition (CVD) allows for artificial synthesis of graphene solely from a ${}^{13}$C precursor, potentially amplifying the influence of the nuclear magnetic moments. In this work we study the effect of hyperfine interactions in pure ${}^{13}$C-graphene on its spin transport properties. Using Hanle precession measurements we determine the spin relaxation time and observe a weak increase of ${\tau }_s$ with doping and a weak change of ${\tau }_s$ with temperature, as in natural graphene. For comparison we study spin transport in pure ${}^{12}$C-graphene, also synthesized by CVD, and observe similar spin relaxation properties. As the signatures of hyperfine effects can be better resolved in oblique spin-valve and Hanle configurations, we use finite-element modeling to emulate oblique signals in the presence of a hyperfine magnetic field for typical graphene properties. Unlike in the case of GaAs, hyperfine interactions with ${}^{13}$C nuclei influence electron spin transport only very weakly, even for a fully polarized nuclear system. Also, in the measurements of the oblique spin-valve and Hanle effects no hyperfine features could be resolved. This work experimentally confirms the weak character of hyperfine interactions and the negligible role of ${}^{13}$C atoms in the spin dephasing processes in graphene.

The spin-density-wave ordering in ${\mathrm{Ca}}_{0.5}$${\mathrm{Sr}}_{1.5}$${\mathrm{RuO}}_4$ was studied by polarized and unpolarized neutron diffraction experiments. Below about 20 K ${\mathrm{Ca}}_{0.5}$${\mathrm{Sr}}_{1.5}$${\mathrm{RuO}}_4$ exhibits quasistatic correlations at exactly the incommensurate wave vector at which pure Sr${}_2$RuO${}_4$ shows strong inelastic fluctuations driven by Fermi-surface nesting. The magnetic character of the signal and the orientation of the ordered moments along the $c$ direction can be ascertained by neutron polarization analysis. The magnetic ordering at low temperature is very similar to that found upon minor Ti substitution of ${\mathrm{Sr}}_2$${\mathrm{RuO}}_4$, underlining that this incommensurate spin density wave is the dominant magnetic instability of the unconventional superconductor Sr${}_2$RuO${}_4$.

We construct a hierarchy of exactly solvable spin-1/2 chains with $so{\left(N\right)}_1$ critical points. Our construction is based on the framework of condensate-induced transitions between topological phases. We employ this framework to construct a Hamiltonian term that couples $N$ transverse field Ising chains such that the resulting theory is critical and described by the $so{\left(N\right)}_1$ conformal field theory. By employing spin duality transformations, we then cast these spin chains for arbitrary $N$ into translationally invariant forms that all allow exact solution by the means of a Jordan-Wigner transformation. For odd $N$ our models generalize the phase diagram of the transverse field Ising chain, the simplest model in our hierarchy. For even $N$ the models can be viewed as longer ranger generalizations of the $XY$ chain, the next model in the hierarchy. We also demonstrate that our method of constructing spin chains with given critical points goes beyond exactly solvable models. Applying the same strategy to the Blume-Capel model, a spin-1 generalization of the Ising chain in a generic magnetic field, we construct another critical spin-1 chain with the predicted conformal field theory (CFT) describing the criticality.

Pressure dependence of the Yb valence in YbNiGe${}_3$ has been investigated up to 15.6 GPa at 300 K and up to 7.7 GPa at 17 K by means of x-ray absorption spectroscopy in $L{\alpha }_1$ partial fluorescence yield mode and resonant x-ray emission spectroscopy around the Yb $L_3$ absorption edge. The Yb valence in YbNiGe${}_3$ at ambient pressure strongly fluctuates with the mean valence of $v=2.52\pm 0.01$ at 300 K. The Yb valence rapidly changes toward a trivalent state with pressure up to $\sim$5 GPa, slowly increases up to $\sim$10 GPa, and then reaches a saturated value of $v\sim 2.87$ at 15.6 GPa. The Yb valence at 17 K slightly decreases compared to that at 300 K; $v\sim 2.45$ at ambient pressure and $\sim$$2.72$ at 7.7 GPa. We found that the pressure-induced change in the intensity of a quadrupole component in the x-ray absorption spectra shows the same trend as the Yb valence in YbNiGe${}_3$. In contrast to YbNiGe${}_3$, the Yb valence in YbNiSi${}_3$ is nearly 3 at ambient pressure with almost no temperature dependence.