Electrical-field control of the carrier density of topological insulators (TIs) has greatly expanded the possible practical use of these materials. However, the combination of low-temperature local probe studies and a gate tunable TI device remains challenging. We have overcome this limitation by scanning tunneling microscopy and spectroscopy measurements on in situ molecular-beam epitaxy grown Bi${}_2$Se${}_3$ films on SrTiO${}_3$ substrates with prepatterned electrodes. Using this gating method, we are able to tune the Fermi level of the top surface states within a range of ≈250 meV on a 3-nm-thick Bi${}_2$Se${}_3$ device. We report field effect studies of the surface-state dispersion, band gap, and electronic structure at the Fermi level.

$AA$-stacked bilayer graphene supports Fermi circles in its bonding and antibonding bands, which coincide exactly, leading to symmetry breaking in the presence of electron-electron interactions. We analyze a continuum model of this system in the Hartree-Fock approximation, using a self-consistently screened interaction that accounts for the gap in the spectrum in the broken symmetry state. The order parameter in the ground state is shown to be of the Ising type, involving transfer of charge between the layers in opposite directions for different sublattices. We analyze the Ising phase transition for the system and argue that it continuously evolves into a Kosterlitz-Thouless transition in the limit of vanishing interlayer separation $d$. The transition temperature is shown to depend only on the effective spin stiffness of the system even for $d>0$, and an estimate of its value suggests that the transition temperature is of the order of a few degrees Kelvin.

A hybrid approach to nonequilibrium dynamics of quantum impurity systems is presented. The numerical renormalization group serves as a means to generate a suitable low-energy Hamiltonian, allowing for an accurate evaluation of the real-time dynamics of the problem up to exponentially long times using primarily the time-adaptive density-matrix renormalization group. In particular, by constructing a suitable hybrid chain, discretization errors are essentially eliminated on all time scales of interest. We extract the decay time of the interaction-enhanced oscillations in the interacting resonant-level model and show their quadratic divergence with the interaction strength $U$. Our numerical analysis is in excellent agreement with analytic predictions based on an expansion in $1/U$.

The Kerr effect can arise in a time-reversal invariant dissipative medium that is “gyrotropic,” i.e., one that breaks inversion ($\mathcal{I}$) and all mirror symmetries. Examples of such systems include electron analogs of cholesteric liquid crystals and their descendants, such as systems with chiral charge ordering. We present arguments that the striking Kerr onset seen in the pseudogap phase of a large number of cuprate high-temperature superconductors is an evidence of chiral charge ordering. We discuss additional experimental consequences of a phase transition to a gyrotropic state, including the appearance of a zero-field Nernst effect.

We investigate experimentally and theoretically the resonant emission of single InAs/GaAs quantum dots in a planar microcavity. Due to the presence of at least one residual charge in the quantum dots, the resonant excitation of the neutral exciton is blocked. The influence of the residual doping on the initial quantum dots charge state is analyzed, and the resonant emission quenching is interpreted as a Coulomb blockade effect. The use of an additional nonresonant laser in a specific low power regime leads to the quantum dots neutralization and allows an efficient optical gating of the exciton resonant emission. A detailed population evolution model, developed to describe the quantum dot neutralization and the optical gate effect, perfectly fits the experimental results in the steady-state and dynamical regimes of the optical gate with a single set of parameters. We deduce that ultraslow Auger- and phonon-assisted capture processes govern the quantum dot neutralization with relaxation times in the 1–$100\mu \mathrm{s}$ range. We conclude that the optical gate acts as a very sensitive probe of the quantum dots population relaxation in an unprecedented slow-capture regime.

In-plane resistivity measurements as a function of temperature, magnetic field, and its orientation with respect to the crystallographic $ab$ plane were used to study the upper critical field, $H_{c2}$, of two overdoped compositions of the iron-based superconductor Ba(Fe${}_{1-x}$Ni${}_x$)${}_2$As${}_2$, $x=0.054$ and $x=0.072$. Measurements were performed using precise alignment (with accuracy less than 0.1${}^{\circ }$) of magnetic field with respect to the Fe-As plane. The dependence of the $H_{c2}$ on angle $\theta$ between the field and the $ab$ plane was measured in isothermal conditions in a broad temperature range. We found that the shape of $H_{c2}\left(\theta \right)$ clearly deviates from the Ginzburg-Landau functional form.

We investigate the frequency and temperature dependence of the low-energy electron dynamics in a Landau Fermi liquid with a local self-energy. We show that the frequency and temperature dependencies of the optical conductivity obey universal scaling forms, for which explicit analytical expressions are obtained. For the optical conductivity and the associated memory function, we obtain a number of surprising features that differ qualitatively from the Drude model and are universal characteristics of a Fermi liquid. Different physical regimes of scaling are identified, with marked non-Drude features in the regime where $\hslash \omega \sim k_{\text{B}}T$. These analytical results for the optical conductivity are compared to numerical calculations for the doped Hubbard model within dynamical mean-field theory. For the “universal” low-energy electrodynamics, we obtain perfect agreement between numerical calculations and analytical scaling laws. Both results show that the optical conductivity displays a non-Drude “foot,” which could be easily mistaken as a signature of breakdown of the Fermi liquid, while it actually is a striking signature of its applicability. The aforementioned scaling laws provide a quantitative tool for the experimental identification and analysis of the Fermi-liquid state using optical spectroscopy, and a powerful method for the identification of alternative states of matter, when applicable.

We propose a theory of optically induced currents in dielectrics and wide gap semiconductors exposed to a nonresonant ultrashort laser pulse with a stabilized carrier-envelope phase. To describe strong-field electron dynamics, equations for the density matrix have been solved self-consistently with equations for the macroscopic electric field inside the medium, which we model by a one-dimensional potential. We provide a detailed analysis of physically important quantities (band populations, macroscopic polarization, and transferred charge), which reveals that carrier-envelope phase control of the electric current can be interpreted as a result of quantum-mechanical interference of multiphoton excitation channels. Our numerical results are in good agreement with experimental data.

We calculate the Andreev conductance of a superconducting ring interrupted by a flux-biased Josephson junction, searching for electrical signatures of circulating edge states. Two-dimensional pair potentials of spin-singlet $d$-wave and spin-triplet $p$-wave symmetry support, respectively, (chiral) Dirac modes and (chiral or helical) Majorana modes. These produce $h/e$-periodic magnetoconductance oscillations of amplitude $\simeq (e^2/h)N^{-1/2}$, measured via an $N$-mode point contact at the inner or outer perimeter of the grounded ring. For Dirac modes the oscillations in the two contacts are independent, while for an unpaired Majorana mode they are phase locked by a topological phase transition at the Josephson junction.

Dependence of relaxor behavior of incompletely filled tetragonal tungsten bronze uniaxial relaxor ferroelectric calcium barium niobate (Ca${}_x$Ba${}_{1-x}$Nb${}_2$O${}_6$, CBN-$x$) on its composition was investigated by varying Ca/Ba ratio (0.18 ≤ $x$ ≤ 0.35) and studying its thermal and elastic properties. Recently, we have reported the relaxor behavior CBN-28 with the evidence of the existence of the Burns temperature $T_{\mathrm{B}}$, and the intermediate characteristic temperature $T^{*}$ [Pandey et al., Phys. Rev. B 84, 174102 (2011)]. In this work, we show that the dynamics of polar nanoregions (and hence the relaxor behavior) strongly varies with the Ca/Ba ratio. Evidence is found for a more pronounced relaxor behavior with increasing $x$. The Curie temperature and the Burns temperature are also very sensitive to the composition, whereas the characteristic temperature $T^{*}$ appears unaffected from the Ca/Ba ratio. The bonding interaction has been explained on the basis of bulk modulus, Poisson's ratio, and deviation from Cauchy relations. Presented results open the perspective to understand the variation of relaxor behavior of CBN-$x$ $(\sim 0.18\le x\le \sim 0.35)$ above Curie temperature.

It is well known that screw dislocation motion dominates the plastic deformation in body-centered-cubic metals at low temperatures. The nature of the nonplanar structure of screw dislocations gives rise to high lattice friction, which results in strong temperature and strain rate dependence of plastic flow. Thus the nature of the Peierls potential, which is responsible for the high lattice resistance, is an important physical property of the material. However, current empirical potentials give a complicated picture of the Peierls potential. Here, we investigate the nature of the Peierls potential using density functional theory in the bcc transition metals. The results show that the shape of the Peierls potential is sinusoidal for every material investigated. Furthermore, we show that the magnitude of the potential scales strongly with the energy per unit length of the screw dislocation in the material.

The ac admittance of solar cells under illumination is investigated under open-circuit conditions. Open-circuit conditions are imposed by inserting a probe capacitor into the circuit. The capacitance and conductance of the cells are investigated as function of frequency and continuous illumination intensity. Results are compared with numerical and analytical modeling of charge recombination and transport. In bulk heterojunction solar cells with [6,6]-Phenyl-C${}_{61}$(C${}_{71}$)-butyric acid methyl ester as acceptor and poly(3-hexylthiophene) or poly[2-methoxy-5-(2${}^{\prime }$-ethylhexyloxy)-p-phenylene vinylene] as electron donor, the high-frequency capacitance $C$ and conductance $G$ follow a power-law dependence on intensity of white light $I$, with $G$($I$) $\propto$ $I$${}^{3/4}$ and $C$($I$) $\propto$ $I$${}^{1/4}$. The modeling shows that these dependencies can be explained in terms of space-charge-limited current in combination with Langevin type recombination of carriers. For poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b${}^{\prime }$]dithiophene-2,6-diyl]] the capacitance shows a weaker dependence on intensity, indicating fast recombination of photogenerated carriers. Results indicate that the fill factor of relatively well performing polymer solar cells can still be limited by space charge effects and can be improved by enhancing the charge carrier mobility or by reducing the bimolecular Langevin recombination.

For a quantum state, or classical harmonic normal mode, of a system of spatial periodicity “$R$,” Bloch character is encoded in a wave vector “$K$.” One can ask whether this state has partial Bloch character “$k$” corresponding to a finer scale of periodicity “$r$.” Answering this is called “unfolding.” A theorem is proven that yields a mathematically clear prescription for unfolding, by examining translational properties of the state, requiring no “reference states” or basis functions with the finer periodicity ($r,k$). A question then arises: How should one assign partial Bloch character to a state of a finite system? A slab, finite in one direction, is used as the example. Perpendicular components $k_z$ of the wave vector are not explicitly defined, but may be hidden in the state (and eigenvector $|i\rangle$). A prescription for extracting $k_z$ is offered and tested. An idealized silicon (111) surface is used as the example. Slab unfolding reveals surface-localized states and resonances which were not evident from dispersion curves alone.

We study a pseudogap Anderson-Holstein model of a magnetic impurity level that hybridizes with a conduction band whose density of states vanishes in power-law fashion at the Fermi energy, and couples, via its charge, to a nondispersive bosonic mode (e.g., an optical phonon). The model, which we treat using poor-man's scaling and the numerical renormalization group, exhibits quantum phase transitions of different types depending on the strength of the impurity-boson coupling. For weak impurity-boson coupling, the suppression of the density of states near the Fermi energy leads to quantum phase transitions between strong-coupling (Kondo) and local-moment phases. For sufficiently strong impurity-boson coupling, however, the bare repulsion between a pair of electrons in the impurity level becomes an effective attraction, leading to quantum phase transitions between strong-coupling (charge Kondo) and local-charge phases. Even though the Hamiltonian exhibits different symmetries in the spin and charge sectors, the thermodynamic properties near the two types of quantum phase transition are closely related under spin-charge interchange. Moreover, the critical responses to a local magnetic field (for small impurity-boson coupling) and to an electric potential (for large impurity-boson coupling) are characterized by the same exponents, whose values place these quantum-critical points in the universality class of the pseudogap Anderson model. One specific case of the pseudogap Anderson-Holstein model may be realized in a double-quantum-dot device, where the quantum phase transitions manifest themselves in the finite-temperature linear electrical conductance.

We have performed systematic studies of the lattice dynamics in $L$FeAsO ($L=\text{La}$, Ce, Pr, Nd, Sm) in the parent and in the $\sim$10% F-doped compounds as a function of pressure and temperature. We have found that the modifications in the partial Fe density of phonon states are mainly governed by the Fe-As bond length. The change of this bond length explains the change of the Fe density of phonon states above 25 meV. We further observe anomalies in the behavior of the phonon mode near 16 meV. In the parent phase, this mode softens anomalously upon cooling through the structural phase transition. Upon F doping, this mode hardens indicating a strong electron-phonon coupling. This suggests that the corresponding phonons play an important role in the competition between superconductivity and magnetism in these materials.

The spin waves in the multi-k antiferromagnet USb soften and become quasielastic well below the antiferromagnetic ordering temperature $T_N$. This occurs without a magnetic or structural transition. It has been suggested that this change is in fact due to dephasing of the different multi-k components: a switch from 3-k to 1-k behavior. In this work, we use inelastic neutron scattering with tridirectional polarization analysis to probe the quasielastic magnetic excitations and reveal that the 3-k structure does not dephase. More surprisingly, the paramagnetic correlations also maintain the same clear phase correlations well above $T_N$ (up to at least $1.4T_N$).

We study electronic transport in junctions consisting of a superconductor electrode and two ferromagnet (F) leads in which crossed Andreev reflections (CAR) and elastic cotunnelings are accommodated. We model the system using an extended Blonder-Tinkham-Klapwijk treatment with a key modification that accounts for spin-dependent interfacial barriers (SDIB). We compute current-voltage relations as a function of parameters characterizing the SDIB, magnetization in the F leads, geometry of the junction, and temperature. Our results reveal a rich range of significantly altered physics due to a combination of interfering spin-dependent scattering processes and population imbalance in the ferromagnets, such as a significant enhancement in CAR current and a sign change in the relative difference between resistance of two cases having a antiparallel or parallel alignment of the magnetization in the F leads, respectively. Our model accounts for the surprising experimental findings of positive relative resistance by M. Colci et al. [Phys. Rev. B 85, 180512(R) (2012)] as well as previously measured negative relative resistance results, both within sufficiently large parameter regions.

We investigate the switching criteria of magnetic vortices in micron-sized Permalloy squares. The vortices are excited by high frequency magnetic fields. Continuous core reversal is demonstrated for a wide range of frequencies and amplitudes of excitation by ferromagnetic absorption spectroscopy and for selected frequencies and amplitudes with time-resolved scanning x-ray microscopy. The boundary of this switching regime is derived from the Thiele equation when a critical velocity of $v_{\mathrm{crit}}\approx 250{\mathrm{ms}}^{-1}$ is considered.

Spinel MnV${}_2$O${}_4$ exhibits orbital ordering at $T_c=57$ K accompanied by a structural phase transition from cubic to tetragonal and ferrimagnetic ordering. We measured the temperature dependence of the optical reflectivity spectra on the cleaved surface of a MnV${}_2$O${}_4$ single crystal with controlled twin structures below $T_c$. We found that there are two peaks at 1.6 and 2.3 eV, corresponding to the Mott excitations, in the optical conductivity spectra obtained by the Kramers-Kronig transformation of the reflectivity, and that the two peaks exhibit a similar anisotropic change below $T_c$. Furthermore, we found that these two peaks exhibit a different temperature dependence above $T_c$. We analyzed these results by a model in which the multiplet structures of the $d$ state for the Mott excitation are fully taken into account, and discussed the occupation of the triply degenerate $t_{2g}$ orbitals both below and above $T_c$.

Quasilongitudinal and quasitransverse picosecond strain pulses injected into a ferromagnetic (311) (Ga,Mn)As film induce dynamical shear strain in the film, thereby modulating the magnetic anisotropy and inducing resonant precession of the magnetization at a frequency ∼10 GHz. The modulation of the out-of-plane magnetization component by the quasitransverse strain reaches amplitudes as large as 10$\%$ of the equilibrium magnetization. Our theoretical analysis is in good agreement with the observed results, thus providing a strategy for ultrafast magnetization control in ferromagnetic films by strain pulses.