We investigate the ultrafast response of the bismuth (111) surface by means of time-resolved photoemission spectroscopy. The direct visualization of the electronic structure allows us to gain insights on electron-electron and electron-phonon interaction. Concerning electron-electron interaction, it is found that electron thermalization is fluence dependent and can take as much as several hundreds of femtoseconds at low fluences. This behavior is in qualitative agreement with Landau's theory of Fermi liquids, but the data show deviations from the behavior of a common three-dimensional degenerate electron gas. Concerning electron-phonon interaction, our data allows us to directly observe the coupling of Bloch states to the coherent $A_{1g}$ mode. It is found that surface states are much less coupled to this mode when compared to bulk states. This is confirmed by ab initio calculations of surface and bulk bismuth.

We have investigated the electronic structure of Bi${}_2$Se${}_3$ epitaxial thin films with thicknesses between 15 and 99 quintuple layers (QL) on a Si substrate using a combination of variable angle spectroscopic ellipsometry and infrared transmission spectroscopy. The results we have obtained are consistent with a Fermi level that shifts relative to the conduction band as a function of sample thickness. We also present evidence that the bulk energy gap in these thin films is as much as 0.1 eV smaller than the value of 0.3 eV predicted by band structure calculations and confirmed by photoemission experiments. The thickness dependence of material properties in Bi${}_2$Se${}_3$ observed in this work reveals thickness to be a parameter that can be tuned to control and possibly optimize the bulk properties of Bi${}_2$Se${}_3$ thin films.

The similarities between Hartree-Fock (HF) theory and the density matrix renormalization group (DMRG) are explored. Both methods can be formulated as the variational optimization of a wave-function Ansatz. Linearization of the time-dependent variational principle near a variational minimum allows to derive the random phase approximation (RPA). We show that the nonredundant parameterization of the matrix product state (MPS) tangent space [J. Haegeman, J. I. Cirac, T. J. Osborne, I. Pižorn, H. Verschelde, and F. Verstraete, Phys. Rev. Lett. 107, 070601 (2011)] leads to the Thouless theorem for MPS, i.e., an explicit nonredundant parameterization of the entire MPS manifold, starting from a specific MPS reference. Excitation operators are identified, which extends the analogy between HF and DMRG to the Tamm-Dancoff approximation (TDA), the configuration interaction (CI) expansion, and coupled cluster theory. For a small one-dimensional Hubbard chain, we use a CI-MPS Ansatz with single and double excitations to improve on the ground state and to calculate low-lying excitation energies. For a symmetry-broken ground state of this model, we show that RPA-MPS allows to retrieve the Goldstone mode. We also discuss calculations of the RPA-MPS correlation energy. With the long-range quantum chemical Pariser-Parr-Pople Hamiltonian, low-lying TDA-MPS and RPA-MPS excitation energies for polyenes are obtained.

In this paper, we study the ground-state entanglement properties of finite $XX$ spin-1/2 chains with random couplings using the Jordan-Wigner transformation. We divide the system into two parts and study the reduced density matrices (RDMs) of its subsystems. Due to the free-fermion nature of the problem, the RDMs take the form of that of a free-fermion thermal ensemble. Finding the spectrum of the corresponding entanglement Hamiltonian and corresponding eigenvectors and comparing them with the real-space renormalization-group (RSRG) treatment, we establish the validity of the RSRG approach for entanglement in the limit of strong disorder but also find its limitations when disorder is weak. In the latter case, our paper provides a way to visualize the ``effective spins'' that form long-distance singlet pairs.

We report nuclear magnetic resonance data in the bilayer nickelate La${}_3$Ni${}_2$O${}_6$. This material belongs to a family of low valence nickel oxides with square planar coordination of the Ni ions and is isoelectronic to the high temperature superconducting cuprates. Although the three layer nickelate compound exhibits a spin-state transition accompanied by antiferromagnetic order, the bilayer material shows no phase transition. The NMR data in the bilayer La${}_3$Ni${}_2$O${}_6$ reveal the presence of antiferromagnetic spin fluctuations down to 5 K that are surprisingly similar to those present in the magnetically ordered trilayer La${}_4$Ni${}_3$O${}_8$, suggesting a similar electronic structure in the paramagnetic phase of both compounds.

Continuous-state variable conductivity of nickel oxide/porous silicon nanocomposite thin films indicative of memristance is reported and a mathematical model for the observed behavior is proposed. The variable conductivity phenomena observed are shown to depend on the composition of the films. A generalized composite material conductivity model that explains the time-varying, field-dependent conductivity modulation is developed based on ionic defect migration in the metal oxide nanocrystallites. The model shows excellent agreement with experimental data and allows for estimation of ionic defect mobilities. Multicycle nonvolatility in the devices is shown to be on the order of 1000 s. The tunable conductivity of the nanocomposite is potentially useful in the fabrication of large-scale or high-power analog circuit elements such as memristors, and provides insights into ionic defect migration properties in nanocomposite materials.

When a two-level quantum dot and a plasmonic metal nanoantenna are resonantly coupled by the electromagnetic near-field, the system can exhibit a Fano resonance, resulting in a transparency dip in the optical spectrum of the coupled system. We calculate the nonlinear response of such a system, for illumination both by continuous-wave and ultrafast pulsed lasers, using both a cavity quantum electrodynamics and a semiclassical coupled-oscillator model. For the experimentally relevant case of thermal broadening of the quantum-dot transition (to meV values consistent with $\sim$100 K), we predict that femtosecond pulsed illumination can lead to a reversal of the Fano resonance, with the induced transparency changing into a superscattering spike in the spectrum. This ultrafast reversal is due to a transient change in the phase relationship between the dipoles of the plasmon and exciton. It thus represents a new approach to dynamically control the collective optical properties and coherence of coupled nanoparticle systems.

We propose a theoretical formalism to evaluate the entropy density and entropy flux that takes into account near-field effects, i.e., interference, diffraction, and tunneling of waves. Using the fluctuation-dissipation theorem, expressions for entropy density and entropy flux in a vacuum cavity between planar multilayered media are derived in terms of local density of photons, local density of accessible microscopic states, and velocity of energy transmission. The proposed method is used to determine the maximum work that can be extracted and a thermodynamic limit of the energy conversion efficiency that can be obtained in near-field thermal radiation.

The resonant behavior of vacancy states in graphene is well known but some ambiguities remain concerning, in particular, the nature of the so-called zero energy modes. Other points are not completely elucidated in the case of low but finite vacancy concentration. Here we concentrate on the case of vacancies described within the usual tight-binding approximation. More precisely, we discuss the case of a single vacancy or of a finite number of vacancies in a finite or infinite system.

We investigate the full counting statistics of a single quantum dot strongly coupled to a local phonon and weakly tunnel connected to two metallic electrodes. By employing the generalized nonequilibrium Green-function method and the Lang-Firsov transformation, we derive an explicit analytical formula for the cumulant generating function, which makes one able to identify distinctly the elastic and inelastic contributions to the current and zero-frequency shot noise. We find that at zero temperature, the inelastic effect causes upward steps in the current and downward jumps in the noise at the bias voltages corresponding to the opening of the inelastic channels, which are ascribed to the vibration-induced complex dependencies of electronic self-energies on the energy and bias voltage. More interestingly, the Fano factor exhibits oscillatory behavior with increasing bias voltage and its minimum value is observed to be smaller than one-half.

I here classify of all the fully gapped massive and the gapless phases in bilayer graphene. The effective low-energy theory in bilayer graphene is constructed, and various discrete and continuous symmetries of the noninteracting system is analyzed. Spinless fermions, placed in a quantizing magnetic field, are considered. The quantum anomalous Hall insulator is properly defined. Constructing a particle-hole doubled 16 component Nambu-Dirac spinor, I recognize all the possible fully gapped and the gapless states, which, on the other hand, split the parabolic dispersion into two anisotropic Dirac-like conical ones. A thorough symmetry analysis of all the ordered states is performed. Altogether there are eight insulating and four superconducting phases in bilayer graphene, that can lead to a fully gapped spectrum. Among the gapped superconductors, three are spin singlet, which include uniform $s$-wave and two spatially inhomogeneous, translational symmetry-breaking Kekule superconductors. The triplet pairing exhibits an $f$-wave symmetry. Besides the gapped phases, there are eight semimetallic and eight gapless superconducting states in total, available for fermions to condense into. I also find interesting gapless superconducting states, which break the translational symmetry, dubbed “gapless Fulde-Farrell-Larkin-Ovchinikov” superconductors. I also discuss the role of the Coulomb interaction, and propose various experimental tools to determine the nature of the underlying ordered states.

Molecular beam epitaxy has been employed to obtain Ga${}_{1-x}$Mn${}_x$N films with $x$ up to 10% and Curie temperatures $T_{\text{C}}$ up to 13 K. The magnitudes of $T_{\text{C}}$ and their dependence on $x$, $T_{\text{C}}\left(x\right)\propto x^m$, where $m=2.2\pm 0.2$, are quantitatively described by a tight-binding model of superexchange interactions and Monte Carlo simulations of $T_{\text{C}}$. The critical behavior of this dilute magnetic insulator shows strong deviations from the magnetically clean case ($x=1$), in particular, (i) an apparent breakdown of the Harris criterion, (ii) a nonmonotonic crossover in the values of the susceptibility critical exponent ${\gamma }_{\text{eff}}$ between the high temperature and critical regimes, and (iii) a smearing of the critical region, which can be explained either by the Griffiths effects or by macroscopic inhomogeneities in the spin distribution with a variance $\Delta x=(0.2\pm 0.1)$%.

We present first-principles studies of the thermopower of the organometallic V${}_4$Bz${}_5$ molecule attached between Co electrodes with noncollinear magnetization directions. Different regimes in the formation of the noncollinear magnetic state of the molecule lead to a remarkable nonmonotonous dependence of the thermopower on the angle between the magnetizations of the electrodes. This complex behavior is explained by the resonant properties of the electron transmission. Consequently, the nanocontacts can be utilized for local heating or cooling controlled by the external magnetic field.

Maxwell's equations in curved space time are invariant under electromagnetic duality transformations. We exploit this property to constrain the design parameters of metamaterials used for transformation electromagnetics. We show that a general transformation must be implemented using a dual-symmetric metamaterial. We also show that the spatial part of the coordinate transformation has the same action for both helicity components of the electromagnetic field, while the spatiotemporal part has a helicity dependent effect. Dual-symmetric metamaterials can be designed by constraining the polarizability tensors of their individual constituents, i.e., the meta atoms. We obtain explicit expressions for these constraints. Two families of realistically implementable dual-symmetric meta atoms are discussed, one that exhibits electric-magnetic cross polarizability and one that does not. In simple three-dimensional periodical arrangements of the meta atoms (Bravais lattices), the helicity dependent effect can only be achieved if the meta atoms exhibit nonzero electric-magnetic cross polarizabilities. In our derivations, we find that two dipoles located at the same point, one electric ($\mathbf{p}$) and one magnetic ($\mathbf{m}$), are needed to produce a total field with well-defined helicity equal to $+1$ or $-1$ and that they must be related as $\mathbf{p}=\frac{i}{c}\mathbf{m}$ or $-\frac{i}{c}\mathbf{m}$, respectively.

We study the role of static and dynamical Coulomb correlation effects on the electronic and magnetic properties of individual Mn, Fe, and Co adatoms deposited on the CuN surface. For these purposes, we construct a realistic Anderson model, solve it by using the finite-temperature exact diagonalization method, and compare the calculated one-particle spectral functions with the LDA+$U$ densities of states. In contrast to Mn/CuN and Fe/CuN, the cobalt system tends to form the electronic excitations at the Fermi level. Based on the calculated magnetic response functions, transverse relaxation times for the magnetic moments of impurity orbitals are estimated. To study the effect of the dynamical correlations on the exchange interaction in nanoclusters, we solve the two-impurity Anderson model for the Mn dimer on the CuN surface. It is found that the experimental exchange interaction can be well reproduced by employing $U=3$ eV, which is two times smaller than the value used in static mean-field LDA+$U$ calculations. This suggests an important role of dynamical correlations in the interaction between adatoms on a surface. To estimate the correlated exchange interaction in the general case we derive a simple and transparent analytical expression demonstrating that the renormalization of the electronic spectrum due to dynamical correlations leads to a rescaling of the magnetic interactions compared to density functional results.

A new method is presented for measuring terms in the elastoresistivity tensor $m_{ij}$ of single crystal samples with tetragonal symmetry. The technique is applied to a representative underdoped Fe-arsenide, Ba(Fe${}_{0.975}$Co${}_{0.025}$)${}_2$As${}_2$, revealing an anomalously large and anisotropic elastoresistance in comparison to simple metals. The $m_{66}$ coefficient follows a Curie-Weiss temperature dependence, providing direct evidence that the tetragonal-to-orthorhombic structural phase transition that occurs at $T_s=97.5$ K in this material is not the result of a true-proper ferroelastic transition. Rather, the material suffers a pseudoproper transition for which the lattice strain is not the primary order parameter.

A series of (Zn,Co)O layers with Co contents $x$ up to 40$\%$ grown by atomic layer deposition have been investigated. All structures deposited at 160${}^{\circ }$C show magnetic properties specific to II-VI dilute magnetic semiconductors with localized spins $S=3/2$ coupled by strong but short-range antiferromagnetic interactions resulting in low-temperature spin-glass freezing for $x=0.16$ and 0.4. At higher growth temperature (200${}^{\circ }$C) metallic Co nanocrystals precipitate in two locations giving rise to two different magnetic responses: (i) a superparamagnetic contribution coming from volume disperse nanocrystals; (ii) a ferromagneticlike behavior brought about by nanocrystals residing at the (Zn,Co)O/substrate interface. It is shown that the dipolar coupling within the interfacial two-dimensional dense dispersion of nanocrystals is responsible for the ferromagneticlike behavior.

We theoretically study how reflection asymmetry affects the neutral exciton diamagnetic coefficient in self-assembled InGaAs/GaAs semiconductor wobbled quantum rings. The previously proposed mapping method is used to simulate the exciton wave function and energy in the rings. The description is suited to clarify the important question of the exciton diamagnetic coefficient stability in the rings with broken reflection symmetry. Our simulation results confirm that the exciton wave function of the reflection symmetrical (balanced) wobbled ring is distributed equally over two potential valleys corresponding to the hills in the ring's shape. At the same time, even a very small reflectional imbalance in the geometry and (or) material content of the wobbled rings destroys the ringlike shape of the exciton wave function and causes the localization of the function in one of the potential valleys of the ring (dotlike shape of the exciton wave function). This leads to a rapid decrease of the exciton diamagnetic coefficient. Our calculation results are in good agreement with recent experimental observations.

The valley-orbit coupling in a few-electron Si quantum dot is expected to be a function of its occupation number $N$. We study the spectrum of multivalley Si quantum dots for $2\le N\le 4$, showing that, counterintuitively, electron-electron interaction effects on the valley-orbit coupling are negligible. For $N=2$ they are suppressed by valley interference, for $N=3$ they vanish due to spinor overlaps, and for $N=4$ they cancel between different pairs of electrons. To corroborate our theoretical findings, we examine the experimental energy spectrum of a few-electron metal-oxide-semiconductor quantum dot. The measured spin-valley state filling sequence in a magnetic field reveals that the valley-orbit coupling is definitively unaffected by the occupation number.