The SnPc/PTCDA/Ag(111) heteromolecular interface and the SnPc/Ag(111) interface are studied by photoelectron spectroscopy of the core-level and the valence regime as well as by work-function measurements. Accordingly, the interaction between the first and the second adsorbate monolayer is comparatively weak and noncovalent in contrast to what is found for the interaction between the substrate and the first adsorbate layer. Furthermore, a rigid level shift is observed for the SnPc spectra of the heteromolecular film, which decreases with increasing SnPc coverage, similar to what is known for Schottky contacts. It will be demonstrated that the built-in electric field at the interface can be explained by the formation of an interface dipole, which extends over several adsorbate monolayers. Moreover, the comparison with the SnPc/Ag(111) film suggests that the interface dipole can be altered by the interlayer.

We demonstrate optical single-electron spin initialization, storage, and readout in a single self-assembled InGaAs quantum dot using a spin memory device. Single-electron spin relaxation is monitored over time scales exceeding $\ge 30\mu \text{s}$, defined only by extrinsic experimental parameters such as the optical detection efficiency. The selective generation of a single electron in the dot is performed by resonant optical excitation and subsequent partial exciton ionization; the hole is removed from the dot while the electron remains stored. When subject to a magnetic field applied in Faraday geometry, we show how the spin of the electron can be prepared with a well-defined spin projection relative to the light propagation direction simply by controlling the voltage applied to the gate electrode. The spin is stored then in the dot before being read out using an optical implementation of spin to charge conversion, whereby the spin projection of the electron is mapped onto a more robust variable, the charge state of the dot. After spin to charge conversion, we show how the charge occupancy can be repeatedly and nonperturbatively measured by pumping a luminescence recycling transition. The approach is shown to provide a readout signal ${10}^4$ times stronger per spin when compared to previous methods. In combination with spin manipulation using the optical Stark effect or microwaves, our approach provides an ideal basis for probing spin coherence in single self-assembled quantum dots over long-time scales and the development of methods for coherent spin control.

The total momentum transfer from fast electron beams, like those employed in scanning transmission electron microscopy (STEM), to plasmonic nanoparticles is calculated. The momentum transfer is obtained by integrating the electromagnetic forces acting on the particles over time. Numerical results for single and dimer metallic nanoparticles are presented, for sizes ranging between 2 and 80 nm. We analyze the momentum transfer in the case of metallic dimers where the different relevant parameters such as particle size, interparticle distance, and electron beam impact parameter are modified. It is shown that depending on the specific values of the parameters, the total momentum transfer yields a force that can be either attractive or repulsive. The time-average forces calculated for electron beams commonly employed in STEM are on the order of piconewtons, comparable in magnitude to optical forces and are thus capable of producing movement in the nanoparticles. This effect can be exploited in mechanical control of nanoparticle induced motion.

Drift-diffusion theory—which fully describes charge transport in semiconductors—is also universally used to model transport of spin-polarized electrons in the presence of longitudinal electric fields. By transforming spin transit time into spin orientation with precession (a technique called the “Larmor clock”) in current-sensing vertical-transport intrinsic Si devices, we show that spin diffusion (and concomitant spin dephasing) can be greatly enhanced with respect to charge diffusion, in direct contrast to predictions of spin Coulomb-drag diffusion suppression.

We analyze electron-transport data through a Si/SiGe double quantum dot in terms of spin blockade and lifetime-enhanced transport (LET), which is transport through excited states that is enabled by long spin-relaxation times. We present a series of low-bias voltage measurements showing the sudden appearance of a strong tail of current that we argue is an unambiguous signature of LET appearing when the bias voltage becomes greater than the singlet-triplet splitting for the (2,0) electron state. We present eight independent data sets, four in the forward-bias (spin-blockade) regime and four in the reverse-bias (lifetime-enhanced transport) regime and show that all eight data sets can be fit to one consistent set of parameters. We also perform a detailed analysis of the reverse-bias (LET) regime, using transport rate equations that include both singlet and triplet transport channels. The model also includes the energy-dependent tunneling of electrons across the quantum barriers and resonant and inelastic tunneling effects. In this way, we obtain excellent fits to the experimental data, and we obtain quantitative estimates for the tunneling rates and transport currents throughout the reverse-bias regime. We provide a physical understanding of the different blockade regimes and present detailed predictions for the conditions under which LET may be observed.

We re-examine the commonly used density-functional theory plus Hubbard $U$ $\left(\text{DFT}+U\right)$ method for the case of noncollinear magnets. While many studies neglect to explicitly include the exchange-correction parameter $J$, or consider its exact value to be unimportant, here we show that in the case of noncollinear magnetism calculations the $J$ parameter can strongly affect the magnetic ground state. We illustrate the strong $J$ dependence of magnetic canting and magnetocrystalline anisotropy by calculating trends in the magnetic lithium orthophosphate family $\text{Li}M{\text{PO}}_4$ ($M=\text{Fe}$ and Ni) and difluorite family $M{\text{F}}_2$ ($M=\text{Mn}$, Fe, Co, and Ni). Our results can be readily understood by expanding the usual $\text{DFT}+U$ equations within the spinor scheme, in which the $J$ parameter acts directly on the off-diagonal components which determine the spin canting.

We use classical molecular dynamics to evaluate the thermal conductivity $\kappa \left(T\right)$ from the heat-flux correlation $⟨\mathbf{j}\left(0\right)\mathbf{j}\left(t\right)⟩$ for a two-dimensional Lennard-Jones triangular lattice. Our work, which follows Ladd, Moran, and Hoover [Phys. Rev. B 34, 5058 (1986)], finds large deviations from the Eucken-Debye result $\kappa \left(T\right)=A/T$ predicted by the phonon-gas model, even though phonon quasiparticles are fairly well defined. The main source of deviations comes from higher order (anharmonic) terms in the heat-flux operator $\mathbf{j}$. By separating different orders of terms $\mathbf{j}={\mathbf{j}}^{\left(2\right)}+{\mathbf{j}}^{\left(3\right)}+\cdots$, we examine various separate contributions to $\kappa \left(T\right)\approx {\kappa }_{22}+{\kappa }_{23}+\cdots$, both from the harmonic and the anharmonic heat fluxes. We find that ${\kappa }_{22}\left(T\right)\approx A/T$ follows quasiparticle theory fairly well but important terms from ${\kappa }_{23}$ and ${\kappa }_{24}$ are independent of $T$ in the classical (high $T$) limit. We use diagrammatic perturbation theory applied to the quantum Kubo formula, to check and explain the $T$ dependence found numerically from anharmonic heat fluxes. We also demonstrate the importance of vertex correction in obtaining the correct quasiparticle coefficient of $1/T$.

The dilute Kondo compound, ${\text{Th}}_{1-x}{\text{U}}_x{\text{Ru}}_2{\text{Si}}_2$, displays non-Fermi-liquid behavior but no zero-point entropy; it thus appears to elude description by known single-ion models. It may also provide a clue to the underlying local degrees of freedom in its dense counterpart, ${\text{URu}}_2{\text{Si}}_2$. Here we use high-resolution magnetization studies to cross-check the thermodynamic consistency of previous experiments. Measurement of the field dependence of the temperature scale, $T_F\left(H\right)$, associated with Fermi-liquid behavior probes the nature of the underlying impurity fixed point. We find that $T_F\left(H\right)$ grows linearly with applied field, in contrast to the quadratic form expected for the two-channel Kondo model. We use a scaling argument to show that the observed behavior of $T_F\left(H\right)$ is consistent with the absence of zero-point entropy, suggesting novel impurity behavior in this material. More generally, we suggest the field magnetization as a probe of single-ion physics and make predictions for its behavior in other actinide compounds.

We present results of atomistic empirical pseudopotential calculations and configuration interaction for excitons, positive and negative trions $\left({\text{X}}^{\pm }\right)$, positive and negative quartons $\left({\text{X}}^{2\pm }\right)$, and biexcitons. The structures investigated are laterally aligned InGaAs quantum dot molecules embedded in GaAs under a lateral electric field. The rather simple energetic of excitons becomes more complex in the case of charged quasiparticles but remains tractable. The negative trion spectrum shows four anticrossings in the presently available range of fields while the positive trion shows two. The magnitude of the anticrossings reveals many-body effects in the carrier tunneling process that should be experimentally accessible.

Topological properties of physical systems can lead to robust behaviors that are insensitive to microscopic details. Such topologically robust phenomena are not limited to static systems but can also appear in driven quantum systems. In this paper, we show that the Floquet operators of periodically driven systems can be divided into topologically distinct (homotopy) classes and give a simple physical interpretation of this classification in terms of the spectra of Floquet operators. Using this picture, we provide an intuitive understanding of the well-known phenomenon of quantized adiabatic pumping. Systems whose Floquet operators belong to the trivial class simulate the dynamics generated by time-independent Hamiltonians, which can be topologically classified according to the schemes developed for static systems. We demonstrate these principles through an example of a periodically driven two-dimensional hexagonal lattice tight-binding model which exhibits several topological phases. Remarkably, one of these phases supports chiral edge modes even though the bulk is topologically trivial.

We show that an ordinary semiconducting thin film with spin-orbit coupling can, under appropriate circumstances, be in a quantum topologically ordered state supporting exotic Majorana excitations which follow non-Abelian statistics. The key to the quantum topological order is the coexistence of spin-orbit coupling with proximity-induced $s$-wave superconductivity and an externally induced Zeeman coupling of the spins. For the Zeeman coupling below a critical value, the system is a nontopological (proximity-induced) $s$-wave superconductor. However, for a range of Zeeman coupling above the critical value, the lowest energy excited state inside a vortex is a zero-energy Majorana fermion state. The system, thus, has entered into a non-Abelian $s$-wave superconducting state via a topological quantum phase transition (TQPT) tuned by the Zeeman coupling. In the topological phase, since the time-reversal symmetry is explicitly broken by the Zeeman term in the Hamiltonian, the edge of the film constitutes a chiral Majorana wire. Just like the $s$-wave superconductivity, the Zeeman coupling can also be proximity induced in the film by an adjacent magnetic insulator. We show this by an explicit model tight-binding calculation for both types of proximity effects in the heterostructure geometry. Here we show that the same TQPT can be accessed by varying the interface transparency between the film and the superconductor. For the transparency below (above) a critical value, the system is a topological (regular) $s$-wave superconductor. In the one-dimensional version of the same structure and for the Zeeman coupling above the critical value, there are localized Majorana zero-energy modes at the two ends of a semiconducting quantum nanowire. In this case, the Zeeman coupling can be induced more easily by an external magnetic field parallel to the wire, obviating the need for a magnetic insulator. We show that, despite the fact that the superconducting pair potential in the nanowire is explicitly $s$ wave, tunneling of electrons to the ends of the wire reveals a pronounced zero-bias peak. Such a peak is absent when the Zeeman coupling is below its critical value, i.e., the nanowire is in the nontopological $s$-wave superconducting state. We argue that the observation of this zero-bias tunneling peak in the semiconductor nanowire is possibly the simplest and clearest experiment proposed so far to unambiguously detect a Majorana fermion mode in a condensed-matter system.

Topological insulators are predicted to present interesting surface transport phenomena but their experimental studies have been hindered by a metallic bulk conduction that overwhelms the surface transport. We show that the topological insulator ${\text{Bi}}_2{\text{Te}}_2\text{Se}$ presents a high resistivity exceeding $1\Omega \text{cm}$ and a variable-range hopping behavior, and yet presents Shubnikov-de Haas oscillations coming from the topological surface state. Furthermore, we have been able to clarify both the bulk and surface transport channels, establishing a comprehensive understanding of the transport in this material. Our results demonstrate that ${\text{Bi}}_2{\text{Te}}_2\text{Se}$ is, to our knowledge, the best material to date for studying the surface quantum transport in a topological insulator.

Electron energy-loss spectroscopy studies have been performed in order to get a deeper insight into the electronic structure and elementary excitations of the two-leg ladder system ${\text{Ca}}_{11}{\text{Sr}}_3{\text{Cu}}_{24}{\text{O}}_{41}$. We find a strong anisotropy of the loss function for momentum transfers along the $a$ and $c$ crystallographic axes, and a remarkable linear plasmon dispersion for a momentum transfer parallel to the legs of the ladders. The investigated spectral features are attributed to localized and delocalized charge-transfer excitations and the charge-carrier plasmon. The charge-carrier plasmon position and dispersion in the long-wavelength limit agree well with expectations based upon the band structure of the two-leg ladder while the observed quasilinear plasmon dispersion might be related to the peculiar properties of underdoped cuprates in general.

We present dynamic nuclear polarization (DNP) in the simplest pseudospin quantum Hall ferromagnet (QHF) of an InSb two-dimensional electron gas with a large $g$ factor using tilted magnetic fields. The DNP-induced amplitude change in a resistance spike of the QHF at large current enables observation of the resistively detected nuclear magnetic resonance of the high nuclear spin isotope ${}^{115}\text{I}\text{n}$ with nine quadrupole splittings. Our results demonstrate the importance of domain structures in the DNP process. The nuclear spin relaxation time $T_1$ in this QHF was relatively short $\left(\sim 120\text{s}\right)$ and almost temperature independent.

X-ray magnetic circular dichroism provides evidence of an out-of-plane spin moment in undoped and doped cuprates. In ${\text{La}}_2{\text{CuO}}_4$ this moment is related to the canting of the antiferromagnetically ordered ${\text{Cu}}^{2+}$ spins caused by the Dzyaloshinskii-Moriya interaction within the ${\text{CuO}}_2$ planes. This canting gives rise to the well-known weak ferromagnetism in high magnetic fields. We find a similar behavior in doped compounds, both in the normal and in the superconducting state, but with a different temperature dependence typical of paramagnetic systems. This result suggests that, together with short-range in-plane antiferromagnetic correlations, the Dzyaloshinskii-Moriya interaction survives up to optimal doping and in the superconducting state.

We study two-electron singlet-triplet relaxation of donor-bound electrons in Silicon. Hyperfine interaction of the electrons with the phosphorus (P) nuclei, in combination with the electron-phonon interaction, lead to relaxation of the triplet states. Within the Heitler-London and effective-mass approximations, we calculate the triplet relaxation rates in the presence of an applied magnetic field. This relaxation mechanism affects the resonance peaks in current electron-spin-resonance experiments on P dimers. Moreover, the estimated time scales for the spin decay put an upper bound on the gate pulses needed to perform fault-tolerant two-qubit operations in donor-spin-based quantum computers. We have found the optimal regimes though, which mitigate this relaxation mechanism, yet permit sufficiently fast two-qubit operations.

The recent discovery of nonlinear current-dependent magnetoresistance in dual spin valve devices [A. Aziz, O. P. Wessely, M. Ali, D. M. Edwards, C. H. Marrows, B. J. Hickey, and M. G. Blamire, Phys. Rev. Lett. 103, 237203 (2009)] opens up the possibility for distinct physics which extends the standard model of giant magnetoresistance. When the outer ferromagnetic layers of a dual spin valve are antiparallel, the resulting accumulation of spin in the middle ferromagnetic layer strongly modifies its bulk and interfacial spin asymmetry and resistance. Here, we report experimental evidence of the role of bulk spin accumulation in this nonlinear effect and show that interfacial spin accumulation alone cannot account for the observed dependence of the effect on the thickness of the middle ferromagnetic layer. It is also shown that spin torque acting on the middle ferromagnetic layer combined with the nonlinear effect might be useful in understanding the dynamical features associated with the nonlinear behavior.

The itinerant-electron ferromagnet MnSi is a well-known example that shows a transition from Fermi-liquid $\Delta \rho \propto T^2$ to non-Fermi-liquid (NFL) $\Delta \rho \propto T^{3/2}$ behavior when the spiral ferromagnetic transition $T_{\text{c}}\approx 29\text{K}$ at ambient pressure is suppressed to zero by the application of hydrostatic pressures above $P_{\text{c}}\approx 14.6\text{kbar}$. Several experimental probes have been employed to reveal the intriguing properties near $P_{\text{c}}$. In this paper, we report the temperature dependence of thermoelectric power $S\left(T\right)$ under hydrostatic pressures up to 20 kbar on a single crystal of MnSi. At pressures close to $P_{\text{c}}$, we observed at low temperatures an unusual enhancement of $S\left(T\right)$, which can be described well with the relationship $S/T\propto \text{ln}\left(1/T\right)$, a formula that has been proposed to describe a system as a quantum critical point (QCP) is approached. The relationship has previously been observed in systems close to a magnetic QCP, for example, ${\text{La}}_{1.6-x}{\text{Nd}}_{0.4}{\text{Sr}}_x{\text{CuO}}_4$ $\left(x=0.24\right)$. The enhancement of thermoelectric power in MnSi occurs at a temperature above the NFL phase and over a broad pressure range around $P_{\text{c}}$.

Understanding the complex glassy phenomena, which accompany polaron formation in optimally doped manganites (ODMs) is a cumbersome issue with many unexplained perspectives. Here, on the basis of ${}^{139}\text{L}\text{a}$ and ${}^{55}\text{M}\text{n}$ nuclear magnetic resonance (NMR) measurements, performed in the temperature range 80–900 K we show that glass freezing, observed in the paramagnetic (PM) phase of ODM ${\text{La}}_{0.67}{\text{Ca}}_{0.33}{\text{MnO}}_3$, is not a random uncorrelated process but the signature of the formation of a genuine spin-glass state, which for $T<T_c$ consolidates with the ferromagnetic (FM) state into a single thermodynamic phase. Comparison with NMR measurements performed on ${\text{La}}_{1-x}{\text{Ca}}_x{\text{MnO}}_3$ systems for $0.0\le x\le 0.41$ and ODM ${\text{La}}_{0.70}{\text{Sr}}_{0.30}{\text{MnO}}_3$, demonstrates the key role played by the local lattice distortions, which control (i) the stability of the spin-glass phase component and (ii) the kind (first or second order) of the PM-FM phase transition. The experimental results are in agreement with the predictions of the compressible random bond-random field Ising model, where consideration of a strain field induced by lattice distortions is shown to invoke at $T_c$ a discontinuous first-orderlike change in both the FM and the “glassy” Edwards-Anderson order parameters.

We provide detailed calculation of the ac conductivity in the case of $1/r$ Coulomb interacting massless Dirac particles in graphene in the collisionless limit when $\omega ⪢T$. The analysis of the electron self-energy, current vertex function, and polarization function, which enter into the calculation of physical quantities including the ac conductivity, is carried out by checking the Ward-Takahashi identities associated with the electrical charge conservation and making sure that they are satisfied at each step. We adopt a variant of the dimensional regularization of Veltman and ’t Hooft by taking the spatial dimension $D=2-ϵ$ for $ϵ>0$. The procedure adopted here yields a result for the conductivity correction which, while explicitly preserving charge conservation laws, is nevertheless different from the results reported previously in literature.