Abstract: Superfluidity has been predicted and now observed in a number of different electron-hole double-layer semiconductor heterostructures. In some of the heterostructures, such as GaAs and Ge-Si electron-hole double quantum wells, there is a strong mismatch between the electron and hole effective masses. We systematically investigate the sensitivity to unequal masses of the superfluid properties and the self-consistent screening of the electron-hole pairing interaction. We find that the superfluid properties are insensitive to mass imbalance in the low density BEC regime of strongly-coupled boson-like electron-hole pairs. At higher densities, in the BEC-BCS crossover regime of fermionic pairs, we find that mass imbalance between electrons and holes weakens the superfluidity and expands the density range for the BEC-BCS crossover regime. This permits screening to kill the superfluid at a lower density than for equal masses.

Abstract: Quantum transport calculations are conducted using nonequilibrium Green's functional formalism to study the effect of halogenation on the electronic transport properties of aromatic S-(C6H5)(2)X and alkanethiolate S-(CH2)(11)X molecules (with X = H, F, Cl, Br, or I) sandwiched between gold electrodes. In terms of conductance, both molecules show the same dependence on the halogen terminal groups despite their different electronic nature. For example, fluorination results in a reduction of the current by almost an order of magnitude, whereas iodine substitution leads to larger current as compared to the reference system (i.e. hydrogen termination). Regarding the asymmetry in the current-voltage characteristics, halogenation reduces the rectification level for the aromatic molecule with the smallest asymmetry for iodine termination. However, in the case of alkanethiolate molecule, halogen substitution increases the current rectification except for fluorination. A physical explanation of these results is obtained from the analysis of the behavior of the density of states, transmission spectra and transmission eigenstates. These findings are of practical importance in exploring the potential of halogenation for creating functional molecular self-assemblies on metallic substrates.

Abstract: The buckling of graphene nano-ribbons containing a grain boundary is studied using atomistic simulations where free and supported boundary conditions are invoked. We consider the buckling transition of two kinds of grain boundaries with special symmetry. When graphene contains a large angle grain boundary with theta = 21.8 degrees, the buckling strains are larger than those of perfect graphene when the ribbons with free (supported) boundary condition are subjected to compressive tension parallel (perpendicular) to the grain boundary. This is opposite for the results of theta = 32.2 degrees. The shape of the deformations of the buckled graphene nanoribbons depends on the boundary conditions, the presence of the particular used grain boundaries, and the direction of applied in-plane compressive tension. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3692573]

Abstract: Using density-functional theory in combination with the nonequilibrium Green's function formalism, we study the effect of the crystal lattice structure of organometallic perovskite CH3NH3PbI3 on its electronic transport properties. Both dispersive interactions and spin-orbit coupling are taken into account in describing structural and electronic properties of the system. We consider two different phases of the material, namely the orthorhombic and cubic lattice structures, which are energetically stable at low (< 160 K) and high (> 330 K) temperatures, respectively. The sizable geometrical differences between the two structures in term of lattice parameters, PbI6 octahedral tilts, rotation and deformations, have considerable impact on the transport properties of the material. For example, at zero bias and for all considered electron energies, the cubic phase has a larger transmission than the orthorhombic one, although both show similar electronic densities of states. Depending on the applied voltage, the current in the cubic system can be several orders of magnitude larger as compared to the one obtained for the orthorhombic sample. We attribute this enhancement in the transmission to the presence of extended states in the cubic phase due to the symmetrically shaped and ordered PbI6 octaherdra. (C) 2015 Elsevier B.V. All rights reserved.

Abstract: We investigate the influence of an external perpendicular magnetic field on the lowest-energy states of an electron bound to a donor which is located near a semiconductor-metal interface. The problem is treated within the effective mass approach and the lowest-energy states are obtained through (1) the “numerically exact” finite element method, and (2) a variational approach using a trial wave function where all image charges that emerge due to the presence of the metallic gate are taken into account. The trial wave functions are constructed such that they reduce to an exponential behavior for sufficiently small magnetic fields and become Gaussian for intermediate and large magnetic fields. The average electron-donor distance can be controlled by the external magnetic field. We find that the size of the 2p(z) state depends strongly on the magnetic field when the donor is close to the interface, showing a nonmonotonic behavior, in contrast with the ground and the other excited states. DOI: 10.1103/PhysRevB.87.075313

Abstract: Using the effective continuum model, the electron energy spectrum of gated bilayer graphene with a step-like region of decoupled graphene layers at the edge of the sample is studied. Different types of coupled-decoupled interfaces are considered, i.e., zigzag (ZZ) and armchair junctions, which result in significant different propagating states. Two non-valley-polarized conducting edge states are observed for ZZ type, which are mainly located around the ZZ-ended graphene layers. Additionally, we investigated both BA-BA and BA-AB domain walls in the gated bilayer graphene within the continuum approximation. Unlike the BA-BA domain wall, which exhibits gapped insulating behaviour, the domain walls surrounded by different stackings of bilayer regions feature valley-polarized edge states. Our findings are consistent with other theoretical calculations, such as from the tight-binding model and first-principles calculations, and agree with experimental observations. Published by AIP Publishing.

Abstract: Nonequilibrium self-organized patterns formed by particles interacting through competing range interaction are driven over a substrate by an external force. We show that, with increasing driving force, the preexisted static patterns evolve into dynamic patterns either via disordered phase or depinned patterns or via the formation of nonequilibrium stripes. Strikingly, the stripes are formed either in the direction of the driving force or in the transverse direction, depending on the pinning strength. The revealed dynamical patterns are summarized in a dynamical phase diagram.

Abstract: Phase-slip centers/lines and hot spots are the main mechanisms for dissipation in current-carrying superconducting thin films. The pulsed-current method has recently been shown to be an effective tool in studying the dynamics of phase-slip centers and their evolution to hot spots. We use the time-dependent Ginzburg-Landau theory in the study of the dynamics of the superconducting condensate in superconducting strips under external current and zero external magnetic field. We show that both the flux-flow state (i.e., slow-moving vortices) and the phase-slip line state (i.e., fast-moving vortices) are dynamically stable dissipative units with temperature smaller than the critical one, whereas hot spots, which are localized normal regions where the local temperature exceeds the critical value, expand in time, resulting ultimately in a complete destruction of the condensate. The response time of the system to abrupt switching on of the overcritical current decreases with increasing both the value of the current (at all temperatures) and temperature (for a given value of the applied current). Our results are in good qualitative agreement with experiments we have conducted on Nb thin strips.

Abstract: The spectral properties of a classical two-dimensional (2D) cluster of charged particles which are confined by a quadratic potential are calculated. Using the method of Newton optimization we obtain the ground state and the metastable states. For a given configuration the eigenvectors and eigenfrequencies for the normal modes are obtained using the Householder diagonalization technique for the dynamical matrix whose elements are the second derivative of the potential energy. For small clusters the lowest excitation corresponds to an intershell rotation. Magic numbers are associated to clusters which are most stable against intershell rotation. For large clusters the lowest excitation is a vortex/anti-vortex pair.

Abstract: Applying the time-dependent Ginzburg-Landau equations, transitions between metastable states of a superconducting ring are investigated in the presence of an external magnetic field. It is shown that if the ring exhibits several metastable states at a particular magnetic field, the transition from one metastable state to another one is governed by both the relaxation time of the absolute value of the order parameter tau(\psi\) and the relaxation time of the phase of the order parameter tau(phi). We found that the larger the ratio tau(\psi\)/tau(phi), the closer the final state will be to the absolute minimum of the free energy, i.e., the thermodynamic equilibrium. The transition to the final state occurs through a subsequent set of single phase slips at a particular point along the ring.