We advance spin noise spectroscopy to an ultrafast tool to resolve high-frequency spin dynamics in semiconductors. The optical nondemolition experiment reveals the genuine origin of the inhomogeneous spin dephasing in $n$-doped GaAs wafers at densities at the metal-to-insulator transition. The measurements prove in conjunction with depth-resolved spin noise measurements that the broadening of the spin dephasing rate does not result from thermal fluctuations or spin-phonon interaction, as suggested previously, but from spatial $g$-factor variations.

The dynamics of magnetic fluctuations in the paramagnetic and antiferroquadrupolarly (AFQ) ordered phases of single crystal ${\text{CeB}}_6$ was investigated by means of muon spin relaxation experiments. Applied magnetic fields up to 2.5 T were used. Since in the AFQ phase the contact hyperfine coupling that contributes to the fluctuating field amplitudes at the three $d$-type interstitial muon sites strongly depends on the temperature and the applied field, as shown by the corresponding Knight-shift results, this feature was taken into account in determining the magnetic fluctuation rates. The extracted correlation time ${\tau }_c$ of the magnetic fluctuations depends only weakly on the applied field, its temperature dependence demonstrates an accelerated slowing down of the spin dynamics in parallel with the development of the AFQ order.

We study a series-coupled double quantum dot in the Kondo regime modeled by the two-impurity Anderson model and find a conduction-band-mediated superexchange interaction that competes with Kondo physics in the strong Coulomb interaction limit. Our numerical renormalization-group results complemented with the higher-order Rayleigh-Schrödinger perturbation theory show that the exchange mechanism leads to clear experimental consequences that can be checked in transport measurements through double quantum dots.

We study the ground-state phase diagram of a one-dimensional spin-$\frac{1}{2}$ easy-plane XXZ model with a ferromagnetic nearest-neighbor (NN) coupling $J_1$ and a competing next-nearest-neighbor (NNN) antiferromagnetic coupling $J_2$ in the parameter range $0<J_2/\left|J_1\right|<0.4$. When $J_2/\left|J_1\right|\lesssim 1/4$, the model is in a Tomonaga-Luttinger liquid phase which is adiabatically connected to the critical phase of the XXZ model of $J_2=0$. On the basis of the effective (sine-Gordon) theory and numerical analyses of low-lying energy levels of finite-size systems, we show that the NNN coupling induces phase transitions from the Tomonaga-Luttinger liquid to gapped phases with either Néel or dimer order. Interestingly, these two types of ordered phases appear alternately as the easy-plane anisotropy is changed toward the isotropic limit. The appearance of the antiferromagnetic (Néel) order in this model is remarkable, as it is strongly unfavored by both the easy-plane ferromagnetic NN coupling and the antiferromagnetic NNN coupling in the classical-spin picture. We argue that emergent trimer degrees of freedom play a crucial role in the formation of the Néel order.

We report the synthesis and characterization of ${\text{Pd}}_{\text{x}}{\text{TiSe}}_2$ via magnetization, resistivity, specific heat measurements, and low-temperature electron diffraction. At very low levels of intercalation, near ${\text{Pd}}_{0.01}{\text{TiSe}}_2$, the commensurate charge density wave transition in the ${\text{TiSe}}_2$ parent compound near 220 K becomes substantially stronger; a distinct transition to a more resistive state is seen at about 80 K. Originally semimetallic, ${\text{TiSe}}_2$ becomes an insulator at low temperatures for $0.01\le x\le 0.06$ in ${\text{Pd}}_{\text{x}}{\text{TiSe}}_2$. Increasing Pd intercalation beyond this level begins to suppress the insulating behavior until a 2 K superconductor is found in the narrow composition range of $0.11\le x\le 0.12$. Beyond $x=0.12$, a normal metallic state (down to 0.3 K) is induced.

We propose a general extended coherent state approach to the qubit (or fermion) and multimode boson coupling systems. The application to the spin-boson model with the discretization of a bosonic bath with arbitrary continuous spectral density is described in detail, and very accurate solutions can be obtained. The quantum phase transition in the nontrivial sub-Ohmic case can be located by the fidelity and the order-parameter critical exponents for the bath exponents $s<1/2$ can be correctly given by the fidelity susceptibility, demonstrating the strength of the approach.

We have performed an extensive study of single crystals of the magnetoelectric ${\text{NdFe}}_3{\left({{}^{11}\text{B}\text{O}}_3\right)}_4$ by means of a combination of single-crystal neutron diffraction and spherical neutron polarimetry. Our investigation did not detect significant deviations at low temperatures from space group $R32$ concerning the chemical structure. With respect to magnetic ordering our combined results demonstrate that in the commensurate magnetic phase below $T_N\approx 30\text{K}$ all three magnetic Fe moments and the magnetic Nd moment are aligned ferromagnetically in the basal hexagonal plane but align antiferromagnetically between adjacent planes. The phase transition to the low-temperature incommensurate (IC) magnetic structure observed at $T_{\text{IC}}\approx 13.5\text{K}$ appears to be continuous. By means of polarized neutron studies it could be shown that in the incommensurate magnetic phase the magnetic structure of ${\text{NdFe}}_3{\left({{}^{11}\text{B}\text{O}}_3\right)}_4$ is transformed into a long-period antiferromagnetic helix with single chirality. Close to the commensurate-incommensurate phase transition third-order harmonics were observed, which in addition indicate the formation of magnetic solitons.

Infrared reflectivity measurements on several 122 iron pnictides reveal the existence of two electronic subsystems. The one gapped due to the spin-density-wave transition in the parent materials, such as ${\text{EuFe}}_2{\text{As}}_2$, is responsible for superconductivity in the doped compounds, such as $\text{Ba}{\left({\text{Fe}}_{0.92}{\text{Co}}_{0.08}\right)}_2{\text{As}}_2$ and $\text{Ba}{\left({\text{Fe}}_{0.95}{\text{Ni}}_{0.05}\right)}_2{\text{As}}_2$. Analyzing the dc resistivity and scattering rate of this contribution, a hidden $T^2$ dependence is found in the normal state. The second subsystem gives rise to incoherent background, present in all 122 compounds, which is basically temperature independent but affected by the superconducting transition.

We present a theory of nonequilibrium quantum criticality in a coupled bilayer system of itinerant electron magnets. The model studied consists of the first layer subjected to an in-plane current and open to an external substrate. The second layer is subject to no direct external drive, but couples to the first layer via short-ranged spin-exchange interaction. No particle exchange is assumed between the layers. Starting from a microscopic fermionic model, we derive an effective action in terms of two coupled bosonic fields which are related to the magnetization fluctuations of the two layers. When there is no interlayer coupling, the two bosonic modes possess different dynamical critical exponents $z$ with $z=2$ $\left(z=3\right)$ for the first (second) layer. This results in multiscale quantum criticality in the coupled system. It is shown that the linear coupling between the two fields leads to a low-energy fixed point characterized by the larger dynamical critical exponent $z=3$. We compute the correlation length in the quantum disordered and quantum critical regimes for both the nonequilibrium case and in thermal equilibrium where the whole system is held at a common temperature $T$. We identify an effective temperature scale $T_{\text{eff}}$ with which we define a quantum-to-classical crossover that is in exact analogy with the thermal equilibrium case but with $T$ replaced by $T_{\text{eff}}$. However, we find that the leading correction to the correlation length in the quantum critical regime scales differently with respect to $T$ and $T_{\text{eff}}$. We also note that the current in the lower layer generates a drift of the magnetization fluctuations, manifesting itself as a parity-breaking contribution to the effective action of the bosonic modes. In this sense, the nonequilibrium drive in this system plays a role which is distinct from $T$ in the thermal equilibrium case. We also derive the stochastic dynamics obeyed by the critical fluctuations in the quantum critical regime and find that they do not fall into the previously identified dynamical universality classes.

We propose a one-step scheme to generate Greenberger-Horne-Zeilinger (GHZ) states for superconducting flux qubits or charge qubits in a circuit QED setup. The GHZ state can be produced within the coherence time of the multiqubit system. Our scheme is independent of the initial state of the transmission line resonator and works in the presence of higher harmonic modes. Our analysis also shows that the scheme is robust to various operation errors and environmental noise.

Rotational misalignment of two stacked honeycomb lattices produces a moiré pattern that is observable in scanning tunneling microscopy as a small modulation of the apparent surface height. This is known from experiments on highly oriented pyrolytic graphite. Here, we observe the combined effect of three-layer moiré patterns in multilayer graphene grown on $\text{SiC}\left(000\overline{1}\right)$. Small-angle rotations between the first and third layer are shown to produce a “double-moiré” pattern, resulting from the interference of moiré patterns from the first three layers. These patterns are strongly affected by relative lattice strain between the layers. We model the moiré patterns as a beat-period of the mismatched reciprocal lattice vectors and show how these patterns can be used to determine the relative strain between lattices, in analogy to strain measurement by optical moiré interferometry.

We have investigated domain clamping in ferroelectric single crystals of ${\text{Bi}}_4{\text{Ti}}_3{\text{O}}_{12}$ (BiT) and $\left({\text{Bi}}_{3.6}{\text{La}}_{0.4}\right){\text{Ti}}_3{\text{O}}_{12}$ (BLT) using polarization measurements along the $a\left(b\right)$ axis and piezoresponse force microscopy (PFM). PFM observations reveal that $90°$ domains are clamped during polarization switching. The crystals of BiT with less oxygen vacancies and BLT exhibited a low volume fraction of the clamped $90°$ domains. Polarization measurements demonstrate that the clamping of $90°$ domains decreases the remanent polarization in BiT crystals. These experimental results and ab initio calculations show that an attractive interaction between $90°$-domain walls and oxygen vacancies is the main origin of the clamping of $90°$ domains.

Recent experimental data on the optical conductivity of niobium-doped ${\text{SrTiO}}_3$ are interpreted in terms of a gas of large polarons with effective coupling constant ${\alpha }_{\text{eff}}\approx 2$. The theoretical approach takes into account many-body effects, the electron-phonon interaction with multiple LO-phonon branches, and the degeneracy and the anisotropy of the $\text{Ti}{t}_{2g}$ conduction band. Based on the Fröhlich interaction, the many-body large polaron theory provides an interpretation for the essential characteristics, except—interestingly—for the unexpectedly large intensity of a peak at $\sim 130\text{meV}$, of the observed optical conductivity spectra of ${\text{SrTi}}_{1-x}{\text{Nb}}_x{\text{O}}_3$ without any adjustment of material parameters.

We present an infrared magneto-optical study of the highly thermoelectric narrow-gap semiconductor ${\text{Bi}}_2{\text{Se}}_3$. Far-infrared and midinfrared (IR) reflectance and transmission measurements have been performed in magnetic fields oriented both parallel and perpendicular to the trigonal $c$ axis of this layered material and supplemented with UV-visible ellipsometry to obtain the optical conductivity ${\sigma }_1\left(\omega \right)$. With lowering of temperature we observe narrowing of the Drude conductivity due to reduced quasiparticle scattering, as well as an increase in the absorption edge due to direct electronic transitions. Magnetic fields $H\parallel c$ dramatically renormalize and asymmetrically broaden the strongest far-IR optical phonon, indicating interaction of the phonon with the continuum free-carrier spectrum and significant magnetoelectric coupling. For the perpendicular field orientation, electronic absorption is enhanced, and the plasma edge is slightly shifted to higher energies. In both cases the direct transition energy is softened in magnetic field.

We consider relaxation of a single-electron spin in a nanotube quantum dot due to its coupling to flexural phonon modes, and identify a new spin-orbit-mediated coupling between the nanotube deflection and the electron spin. This mechanism dominates other spin-relaxation mechanisms in the limit of small energy transfers. Due to the quadratic dispersion law of long-wavelength flexons, $\omega \propto q^2$, the density of states $dq/d\omega \propto {\omega }^{-1/2}$ diverges as $\omega \to 0$. Furthermore, because here the spin couples directly to the nanotube deflection, there is an additional enhancement by a factor of $1/q$ compared to the deformation-potential coupling mechanism. We show that the deflection coupling robustly gives rise to a minimum in the magnetic field dependence of the spin lifetime $T_1$ near an avoided crossing between spin-orbit split levels in both the high- and low-temperature limits. This provides a mechanism that supports the identification of the observed $T_1$ minimum with an avoided crossing in the single-particle spectrum by Churchill et al. [Phys. Rev. Lett. 102, 166802 (2009)].

We describe fluctuating two-dimensional metallic antiferromagnets by transforming to a rotating reference frame in which the electron spin polarization is measured by its projections along the local antiferromagnetic order. This leads to a gauge-theoretic description of an “algebraic charge liquid” involving spinless fermions and a spin $S=1/2$ complex scalar. We propose a phenomenological effective lattice Hamiltonian which describes the binding of these particles into gauge-neutral, electronlike excitations, and describe its implications for the electron spectral function across the entire Brillouin zone. We discuss connections of our results to photoemission experiments in the pseudogap regime of the cuprate superconductors.

It is known that fluctuations in the electrostatic potential allow for metallic conduction (nonzero conductivity in the limit of an infinite system) if the carriers form a single species of massless two-dimensional Dirac fermions. A nonzero uniform mass $\overline{M}$ opens up an excitation gap, localizing all states at the Dirac point of charge neutrality. Here we investigate numerically whether fluctuations $\delta M⪢\overline{M}\ne 0$ in the mass can have a similar effect as potential fluctuations, allowing for metallic conduction at the Dirac point. Our negative conclusion confirms earlier expectations but does not support the recently predicted metallic phase in a random-gap model of graphene.

A joint experimental and theoretical investigation of the spin 1/2 system ${\text{Cu}}_2{\left({\text{PO}}_3\right)}_2{\text{CH}}_2$ suggests a description of this compound as coupled alternating antiferromagnetic Heisenberg chains. Magnetic susceptibility, specific heat, nuclear magnetic resonance, nuclear quadrupole resonance, and high-field magnetization measurements evidence a spin gap of about 25 K. Surprisingly, the leading antiferromagnetic exchange of about 75 K can be assigned by density-functional band-structure calculations to a coupling between the structural ${\text{Cu}}_2{\text{O}}_6$ dimers, whereas the coupling within these dimers is strongly reduced due to sizable ferromagnetic contributions. The coupling within the structural dimers competes with a number of long-range couplings. The present available experimental data can be consistently described in a scenario of coupled alternating chains. The proposed model should be considered as a minimal model for an appropriate description of this compound.

Field-emission patterns from a clean tungsten tip apex induced by femtosecond laser pulses have been investigated. Strongly asymmetric field-emission intensity distributions are observed depending on three parameters: (i) the polarization of the light, (ii) the azimuthal, and (iii) the polar orientation of the tip apex relative to the laser incidence direction. In effect, we have realized an ultrafast pulsed field-emission source with site selectivity of a few tens of nanometers. Simulations of local fields on the tip apex and of electron emission patterns based on photoexcited nonequilibrium electron distributions explain our observations quantitatively. Electron emission processes are found to depend on laser power and tip voltage. At relatively low laser power and high tip voltage, field-emission after two-photon photoexcitation is the dominant process. At relatively low laser power and low tip voltage, photoemission processes are dominant. As the laser power increases, photoemission from the tip shank becomes noticeable.

We have observed the formation of graphene on SiC by Si sublimation in an Ar atmosphere using low-energy electron microscopy, scanning tunneling microcopy, and atomic force microscopy. This work reveals unanticipated growth mechanisms, which depend strongly on the initial surface morphology. Carbon diffusion governs the spatial relationship between SiC decomposition and graphene growth. Isolated bilayer SiC steps generate narrow ribbons of graphene by a distinctive cooperative process, whereas triple bilayer steps allow large graphene sheets to grow by step flow. We demonstrate how graphene quality can be improved by controlling the initial surface morphology to avoid the instabilities inherent in diffusion-limited growth.