We analyze the influence of noise for qubits implemented using a triple quantum dot spin system. We give a detailed description of the physical realization and develop error models for the dominant external noise sources. We use a Davies master equation approach to describe their influence on the qubit. The triple dot system contains two meaningful realizations of a qubit: We consider a subspace and a subsystem of the full Hilbert space to implement the qubit. The main goal of this paper is to test if one of these implementations is favorable when the qubit interacts with realistic environments. When performing the noise analysis, we extract the initial time evolution of the qubit using a Nakajima-Zwanzig approach. We find that the initial time evolution, which is essential for qubit applications, decouples from the long time dynamics of the system. We extract probabilities for the qubit errors of dephasing, relaxation, and leakage. Using the Davies model to describe the environment simplifies the noise analysis. It allows us to construct simple toy models, which closely describe the error probabilities.

The dynamic conductivity $\sigma \left(\omega \right)$ of graphene in the presence of diagonal white noise disorder and quantizing magnetic field $B$ is calculated. We obtain analytic expressions for $\sigma \left(\omega \right)$ in various parametric regimes ranging from the quasiclassical Drude limit corresponding to strongly overlapping Landau levels (LLs) to the extreme quantum limit where the conductivity is determined by the optical selection rules of the clean graphene. The nonequidistant LL spectrum of graphene renders its transport characteristics quantitatively different from conventional 2D electron systems with parabolic spectrum. Since the magneto-oscillations in the semiclassical density of states are anharmonic and are described by a quasicontinuum of cyclotron frequencies, both the ac Shubnikov-de Haas oscillations and the quantum corrections to $\sigma \left(\omega \right)$ that survive to higher temperatures manifest a slow beating on top of fast oscillations with the local energy-dependent cyclotron frequency. Both types of quantum oscillations possess nodes whose index scales as ${\omega }^2$. In the quantum regime of separated LLs, we study both the cyclotron resonance transitions, which have a rich spectrum due to the nonequidistant spectrum of LLs, and disorder-induced transitions, which violate the clean selection rules of graphene. We identify the strongest disorder-induced transitions in recent magnetotransmission experiments. We also compare the temperature and chemical-potential dependence of $\sigma \left(\omega \right)$ in various frequency ranges from the dc limit allowing intra-LL transition only to the universal high-frequency limit where the Landau quantization provides a small $B$-dependent correction to the universal value of the interband conductivity $\sigma =e^2/4\hslash$ of the clean graphene.

We consider a class of models of nonequilibrium electronic Mach-Zehnder interferometers built on integer quantum Hall edges states. The models are characterized by the electron-electron interaction being restricted to the inner part of the interferometer and transmission coefficients of the quantum quantum point contacts, defining the interferometer, which may take arbitrary values from zero to one. We establish an exact solution of these models in terms of single-particle quantities, determinants and resolvents of Fredholm integral operators. In the general situation, the results can be obtained numerically. In the case of strong charging interaction, the operators acquire the block Toeplitz form. Analyzing the corresponding Riemann-Hilbert problem, we reduce the result to certain singular single-channel determinants (which are a generalization of Toeplitz determinants with Fisher-Hartwig singularities) and obtain an analytic result for the interference current (and, in particular, for the visibility of Aharonov-Bohm oscillations). Our results, which are in good agreement with experimental observations, show an intimate connection between the observed “lobe” structure in the visibility of Aharonov-Bohm oscillations and multiple branches in the asymptotics of singular integral determinants.

Using low-temperature scanning tunneling microscopy and spectroscopy, we have studied quantized magnetic fluxes (vortices) that penetrated nanosized superconductors with void structures inside. The void structures act as a pinning center for the vortices, and the trapped vortices are confined and squeezed inside. When multiple vortices are trapped in voids, a cluster of vortices and a giant vortex were formed as a result of the squeezing, depending on the depth and the size of the trapping potential.

The orbital and spin magnetic properties of iron inside metallic and semiconducting carbon nanotubes are studied by means of local x-ray magnetic circular dichroism (XMCD) and bulk superconducting quantum interference device (SQUID). The iron-nanotube hybrids are initially ferrocene filled single-walled carbon nanotubes (SWCNT) of different metallicities. We show that the ferrocene's molecular orbitals interact differently with the SWCNT of different metallicities with no significant XMCD response. At elevated temperatures the ferrocene molecules react with each other to form cementite nanoclusters. The XMCD at various magnetic fields reveal that the orbital and/or spin magnetic moments of the encapsulated iron are altered drastically as the transformation to the 1D clusters takes place. The orbital and spin magnetic moments are both found to be larger in filled semiconducting nanotubes than in the metallic sample. This could mean that the magnetic polarization of the encapsulated material depends on the metallicity of the tubes. From a comparison between the iron 3d magnetic moments and the bulk magnetism measured by SQUID, we conclude that the delocalized magnetisms dominate the magnetic properties of these 1D hybrid nanostructures.

The electronic properties of Cu${}_{0.07}$Bi${}_2$Se${}_3$ have been investigated using Shubnikov–de Haas and optical reflectance measurements. Quantum oscillations reveal a bulk, three-dimensional Fermi surface with anisotropy $k_F^c/k_F^{ab}\approx$ 2 and a modest increase in free-carrier concentration and in scattering rate with respect to the undoped Bi${}_2$Se${}_3$, also confirmed by reflectivity data. The effective mass is almost identical to that of Bi${}_2$Se${}_3$. Optical conductivity reveals a strong enhancement of the bound impurity bands with Cu addition, suggesting that a significant number of Cu atoms enter the interstitial sites between Bi and Se layers or may even substitute for Bi. This conclusion is also supported by x-ray diffraction measurements, where a significant increase of microstrain was found in Cu${}_{0.07}$Bi${}_2$Se${}_3$, compared to Bi${}_2$Se${}_3$.

The electric field ionization of gallium acceptors in germanium was studied by using terahertz-pump–terahertz-probe spectroscopy. As the pump electric field increases, the distinct absorptions due to acceptor transitions centered at 2.0 and 2.2 THz decrease, and simultaneously, a free carrier response emerges in the lower frequency region. These behaviors clearly show that the terahertz-pump pulse ionizes neutral acceptors. The pump electric field dependence of the released hole density is reproduced by a model assuming direct field-assisted tunneling of acceptors.

The Markov entropy decomposition (MED) is a recently proposed, cluster-based simulation method for finite temperature quantum systems with arbitrary geometry. In this paper, we detail numerical algorithms for performing the required steps of the MED, principally solving a minimization problem with a preconditioned Newton's algorithm, as well as how to extract global susceptibilities and thermal responses. We demonstrate the power of the method with the spin-1/2 $XXZ$ model on the 2D square lattice, including the extraction of critical points and details of each phase. Although the method shares some qualitative similarities with exact diagonalization, we show that the MED is both more accurate and significantly more flexible.

The effect of Coulomb correlations in the half-filled Hubbard model of the honeycomb lattice is studied within the dynamical cluster approximation (DCA) combined with exact diagonalization (ED) and continuous-time quantum Monte Carlo (QMC), for unit cells consisting of six-site rings. The important difference between this approach and the previously employed cluster dynamical mean-field theory (CDMFT) is that DCA preserves the translation symmetry of the system, while CDMFT violates this symmetry. As the Dirac cones of the honeycomb lattice are the consequence of perfect long-range order, DCA yields semimetallic behavior at small on-site Coulomb interactions $U$, whereas CDMFT gives rise to a spurious excitation gap even for very small $U$. This basic difference between the two cluster approaches is found regardless of whether ED or QMC is used as the impurity solver. At larger values of $U$, the lack of translation symmetry becomes less important, so that the CDMFT reveals a Mott gap, in qualitative agreement with large-scale QMC calculations. In contrast, the semimetallic phase obtained in DCA persists even at $U$ values where CDMFT and large-scale QMC consistently show Mott-insulating behavior.

The magnetism and electronic structure of Li-doped SnO${}_2$ are investigated using first-principles LDA/LDA$+U$ calculations. We find that Li induces magnetism in SnO${}_2$ when doped at the Sn site but becomes nonmagnetic when doped at the O and interstitial sites. The calculated formation energies show that Li prefers the Sn site as compared with the O site, in agreement with previous experimental works. The interaction of Li with native defects (Sn V${}_{\mathrm{Sn}}$ and O V${}_{\mathrm{O}}$ vacancies) is also studied, and we find that Li not only behaves as a spin polarizer, but also a vacancy stabilizer, i.e., Li significantly reduces the defect formation energies of the native defects and helps the stabilization of magnetic oxygen vacancies. The electronic densities of states reveals that these systems, where the Fermi level touches the conduction (valence) band, are nonmagnetic (magnetic).

We present a detailed theoretical study of transverse exciton-polariton patterns in semiconductor quantum well microcavities. These patterns are initiated by directional instabilities (driven mainly by polariton-polariton scattering) in the uniform pump-generated polariton field and are measured as optical patterns in a transverse plane in the far field. Based on a microscopic many-particle theory, we investigate the spatiotemporal dynamics of the formation, selection, and optical control of these patterns. An emphasis is placed on a previously proposed low-intensity, all-optical switching scheme designed to exploit these instability-driven patterns. Simulations and detailed analyses of simplified and more physically transparent models are used. Two aspects of the problem are studied in detail. First, we study the dependencies of the stability behaviors of various patterns, as well as transition time scales, on parameters relevant to the switching action. These parameters are the degree of built-in azimuthal anisotropy in the system and the switching (control) beam intensity. It is found that if the parameters are varied incrementally, the pattern system undergoes abrupt transitions at threshold parameter values, which are accompanied by multiple-stability and hysteresis behaviors. Moreover, during a real-time switching action, the transient dynamics of the system, in particular, the transition time scale, may depend significantly on the proximity of unstable patterns. The second aspect is a classification and detailed analysis of the polariton scattering processes contributing to the pattern dynamics, giving us an understanding of the selection and control of patterns as results of these processes' intricate interplay. The crucial role played by the (relative) phases of the polariton amplitudes in determining the gains and/or losses of polariton densities in various momentum modes is highlighted. As a result of this analysis, an interpretation of the actions of the various processes in terms of concepts commonly used in classical pattern-forming systems is given.

We first develop a general Green's-function method for the slab geometry, which can handle quite complex conversions between the different polarized radiation modes. Then by using the method, we study the near field thermal coherence properties of a slab of $Z_2$ topological insulator (TI) with a finite thickness. Under a strong enough external magnetic field, the gapless helical Dirac fermions on the TI surfaces can acquire an energy gap larger than the photon energy of the radiation field at the Dirac point. This gapped surface states can couple strongly with the waveguide modes in the bulk of the TI slab, which thus induces an electromagnetic resonance. Exactly due to the resonance, the coherence properties of the thermal radiation field can be modified dramatically. For $Z_2$ topological insulators, the parameter regime with the surface Hall conductivity half integer quantized is unique. We demonstrated that within the parameter regime, for a TI slab of a proper thickness, the thermal radiation energy density can be enhanced very much, and the coherence length of the thermal radiation field can be as long as twice that of SiC.

We present a theoretical study of the resonance fluorescence spectra of an optically driven quantum dot placed near a single metal nanoparticle. The metallic reservoir coupling is calculated for an 8-nm metal nanoparticle using a time-convolutionless master equation approach where the exact photon reservoir function is included using Green function theory. By exciting the system coherently near the nanoparticle dipole mode, we show that the driven Mollow spectrum becomes highly asymmetric due to internal coupling effects with higher-order plasmons. We also highlight the regimes of resonance squeezing and broadening as well as spectral reshaping through light propagation. Our master equation technique can be applied to any arbitrary material system, including lossy inhomogeneous structures, where mode expansion techniques are known to break down.

We present a self-contained theoretical and computational framework for dynamics following photoexcitation in quantum dots near planar interfaces. A microscopic Hamiltonian parametrized by first-principles calculations is merged with a reduced density matrix formalism that allows for the prediction of time-dependent charge and energy transfer processes between the quantum dot and the electrode. While treating charge and energy transfer processes on an equal footing, the nonperturbative effects of sudden charge transitions on the Fermi sea of the electrode are included. We illustrate the formalism with calculations of an InAs quantum dot coupled to the Shockley state on an Au[111] surface and use it to concretely discuss the wide range of kinetics possible in these systems and their implications for photovoltaic systems and tunnel junction devices. We discuss the utility of this framework for the analysis of recent experiments.

Numerical simulations of previous plane shock wave measurements on highly oriented pyrolytic graphite (HOPG), shocked to four peak stresses ranging from 27 to 50 GPa, are presented to address a long-standing question: When is the diamond phase formed in the shock-compressed graphite to diamond transition? A multiphase material description was developed to simulate measured wave profiles in shocked HOPG. Our results showed good agreement with the measured profiles for all four peak stresses and demonstrated the following: Transformation is completed in less than 10 ns, the density and stiffness of the high-pressure phase match that of cubic diamond, and the mechanical response of the diamond phase is nonlinear elastic.

We report a first-principles investigation of magnetic excitations in a compensated half-metallic ferrimagnet using both the adiabatic Heisenberg model and the dynamic spin susceptibility of the electronic system. The combination of half-metallicity and spin compensation of inequivalent magnetic sublattices generates two acoustic magnon modes characterized by linear dispersions with equal spin wave velocities and an asymmetric Landau damping. The difference in the damping is the consequence of the half-metallicity leading to the gap in the energy of the up-to-down electronic spin-flip excitations, whereas down-to-up excitations are gapless. The engineering of the activation energy of the Stoner excitations opens an avenue for the chirality-selective manipulation of the lifetime of magnons.

We compute the field-angle-resolved specific heat and thermal conductivity using realistic model band structures for the heavy-fermion superconductor CeCoIn${}_5$ to identify the gap structure and location of nodes. We use a two-band tight-binding parametrization of the band dispersion as input for the self-consistent calculations in the quasiclassical formulation of the superconductivity. Systematic analysis shows that modest in-plane anisotropy in the density of states and Fermi velocity in tetragonal crystals significantly affects the fourfold oscillations in thermal quantities, when the magnetic field is rotated in the basal plane. The Fermi-surface anisotropy substantially shifts the location of the lines in the $H$-$T$ plane, where the oscillations change sign compared to quasicylindrical model calculations. In particular, at high fields, the anisotropy and sign reversal are found even for isotropic gaps. Our findings imply that a simultaneous analysis of the specific heat and thermal conductivity, with an emphasis on the low-energy sector, is needed to restrict potential pairing scenarios in multiband superconductors. We discuss the impact of our results on recent measurements of the Ce-115 family, namely, Ce$T$In${}_5$ with $T=$ Co, Rh, Ir.

We present a study of the structural and physical properties of directly hole-doped LaFe${}_{1-x}$Mn${}_x$AsO ($x=0.0-0.2$) and the influence of charge compensation by additional F doping in LaFe${}_{0.9}$Mn${}_{0.1}$AsO${}_{1-y}$F${}_y$ ($y=0.1-0.5$). High-quality polycrystalline samples were prepared for Mn-doped LaFe${}_{1-x}$Mn${}_x$AsO and the low doping regime in LaFe${}_{0.9}$Mn${}_{0.1}$AsO${}_{1-y}$F${}_y$ ($y$ $=$ 0.1 and 0.2) using a solid-state metathesis reaction. The unit cell increases upon Mn doping, but decreases again when additional F is inserted. The semiconducting character of LaFe${}_{1-x}$Mn${}_x$AsO decreases with additional F doping. Muon spin relaxation (μSR) measurements reveal short-range magnetic order in LaFe${}_{1-\mathrm{x}}$Mn${}_{\mathrm{x}}$AsO and a suppression of magnetism by additional electron doping with fluoride in LaFe${}_{0.9}$Mn${}_{0.1}$AsO${}_{1-y}$F${}_y$. Superconductivity remains absent even though the electronic preconditions are fulfilled in electron-doped LaFe${}_{0.9}$Mn${}_{0.1}$AsO${}_{1-y}$F${}_y$ at $x$ > 0.1, which is suggestive of effective pair breaking by Mn in this system.

We study theoretically how the dynamics of the resistive state in narrow superconducting channels shunted by an external resistor depends on channel's length $L$, the applied current $j$, and parameter $u$ characterizing the penetration depth of the electric field in nonequilibrium superconductors. We show that changing $u$ dramatically affects both the behavior of the current-voltage characteristics of the superconducting channels and the dynamics of their order parameter. Previously, it was demonstrated that when $u$ is less than the critical value $u_{c1}$, which does not depend on $L$, the phase slip centers appear simultaneously at different spots of the channel. Herewith, for $u>u_{c1}$ these centers arise consecutively at the same place. In our work we demonstrate that there is another critical value for $u$. Actually, if $u$ does not exceed a certain value $u_{c2}$, which depends on $L$, the current-voltage characteristic exhibits the step-like behavior. However, for $u>u_{c2}$ it becomes hysteretic. In this case, with increase of $j$ the steady state, which corresponds to the time-independent distribution of the order parameter along the channel, losses its stability at switching current value $j_{sw}$, and time-periodic oscillations of both the order parameter and electric field occur in the channel. As $j$ sweeps down, the periodic dynamics ceases at certain retrapping current value $j_r<j_{sw}$. Shunting the channel by a resistor increases the value of $j_r$, while $j_{sw}$ remains unchanged. Thus, for some high-enough conductivity of the shunt, $j_r$ and $j_{sw}$ eventually coincide, and the hysteretic loop disappears. We reveal dynamical regimes involved in the hysteresis and discuss the bifurcation transitions between them.

An in-situ investigation from high-resolution neutron powder diffraction data on the structural evolution of TmNiO${}_3$ and YbNiO${}_3$ perovskites across the metal-insulator transition, with $T_{\mathrm{MI}}$ $=$ 596 K and 598 K, respectively, allowed the charge disproportionation effect that these perovskites experience upon electronic localization below $T_{\mathrm{MI}}$ to be followed. In the insulating (semiconducting) regime, the perovskites are monoclinic, space group $P$2${}_1$/$n$, containing two inequivalent Ni1${}^{(3+\sigma )+}$ and Ni2${}^{(3-\sigma )+}$ cations; above T${}_{\mathrm{MI}}$, the samples become orthorhombic, space group Pbnm, with a single site for Ni${}^{3+}$. The ${}^{57}$Fe Mössbauer spectra of iron-doped (at 1.5$\%$) $R$NiO${}_3$ ($R$ $=$ Tm, Yb) samples recorded below $T_{\mathrm{MI}}$ exhibit for Fe1 and Fe2 (replacing Ni1 and Ni2 sites) hyperfine parameters corresponding to large (Ni1O${}_6$) and small (Ni2O${}_6$) octahedra. The remarkable difference between the quadrupole splittings (${\Delta }_1$ ≈ 0.3 mm/s and ${\Delta }_2$ ≈ 0.07 mm/s) of Fe1 and Fe2 sites in $R$Ni${}_{0.985}$Fe${}_{0.015}$O${}_3$ is analyzed. We calculate the lattice contribution to the electric field gradient (EFG) at ${}^{57}$Fe ions, and estimate, by using the experimental ${\Delta }_1$ and ${\Delta }_2$ values, the contributions of the 3$d$, 3$p$, and 2$p$ electrons (overlap distortion and covalence effects). Above $T_{\mathrm{MI}}$, a unique state for iron atoms is observed, upon metallization of the sample.