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 Creator:
 Sheng, Donna, Ma, R., Sheng, L., Zhu, Lijun, and Liu, M.
 Description:
 We numerically study the quantum Hall effect in biased bilayer graphene based on a tightbinding model in the presence of disorder. Integer quantum Hall plateaus with quantized conductivity σxy=νe2/h (where ν is an integer) are observed around the band center due to the split of the valley degeneracy by an opposite voltage bias added to the two layers. The central (n=0) DiracLandau level is also split, which leads to a pronounced ν=0 plateau. This is consistent with the opening of a sizable gap between the valence and conduction bands. The exact spectrum in an open system further reveals that there are no conducting edge states near zero energy, indicating an insulator state with zero conductance. Consequently, the resistivity should diverge at the Dirac point. Interestingly, the ν=0 insulating state can be destroyed by disorder scattering with intermediate strength, where a metallic region is observed near zero energy. In the strongdisorder regime, the Hall plateaus with nonzero ν are destroyed due to the floatup of extended levels toward the band center and higher plateaus disappear first.
 Resource Type:
 Article
 Identifier:
 02955075
 Campus Tesim:
 Northridge
 Creator:
 Zhu, W., Haldane, F.D.M., Sheng, Donna, and Gong, Shoushu
 Description:
 Topological quantum states with nonAbelian Fibonacci anyonic excitations are widely sought after for the exotic fundamental physics they would exhibit, and for universal quantum computing applications. The fractional quantum Hall (FQH) state at a filling factor of ν=12/5 is a promising candidate; however, its precise nature is still under debate and no consensus has been achieved so far. Here, we investigate the nature of the FQH ν=13/5 state and its particlehole conjugate state at 12/5 with the Coulomb interaction, and we address the issue of possible competing states. Based on a largescale densitymatrix renormalization group calculation in spherical geometry, we present evidence that the essential physics of the Coulomb ground state (GS) at ν=13/5 and 12/5 is captured by the k=3parafermion ReadRezayi state (RR3), including a robust excitation gap and the topological fingerprint from the entanglement spectrum and topological entanglement entropy. Furthermore, by considering the infinitecylinder geometry (topologically equivalent to torus geometry), we expose the nonAbelian GS sector corresponding to a Fibonacci anyonic quasiparticle, which serves as a signature of the RR3state at 13/5 and 12/5 filling numbers.
 Resource Type:
 Article
 Identifier:
 00319007
 Campus Tesim:
 Northridge
 Creator:
 Kim, Jinwoong, Mahfouzi, Farzad, and Kioussis, Nicholas G.
 Description:
 A fully quantummechanical description of the precessional damping of a Pt/Co bilayer is presented in the framework of the Keldysh Green's function approach using ab initio electronic structure calculations. In contrast to previous calculations of classical Gilbert damping (αGD), we demonstrate that αGD in the quantum case does not diverge in the ballistic regime due to the finite size of the total spin S. In the limit of S→∞ we show that the formalism recovers the torque correlation expression for αGD which we decompose into spinpumping and spinorbital torque correlation contributions. The formalism is generalized to take into account a selfconsistently determined dephasing mechanism which preserves the conservation laws and allows the investigation of the effect of disorder. The dependence of αGD on Pt thickness and disorder strength is calculated, and the spindiffusion length of Pt and the spin mixing conductance of the bilayer are determined and compared with experiments.
 Resource Type:
 Article
 Identifier:
 10980121
 Campus Tesim:
 Northridge
 Creator:
 Belemuk, A.M., Mikheyenkov, A.V., and Chtchelkatchev, N.M.
 Description:
 We consider the behavior of Fermi atoms on optical superlattices with twowell structure for each node. Fermions on such lattices serve as an analog simulator of the Fermitype Hamiltonian. We derive a mapping between fermion quantum ordering in the optical superlattices and the spinorbital physics developed for degenerate delectron compounds. The appropriate effective spinorbital model appears to be a modification of the KugelKhomskii Hamiltonian. We show how different ground states of this Hamiltonian correspond to particular spinpseudospin arrangement patterns of fermions on the lattice. The dependence of the fermion arrangement on phases of complex hopping amplitudes is illustrated.
 Resource Type:
 Article
 Identifier:
 10502947
 Campus Tesim:
 Northridge
 Creator:
 Udalov, Oleg G. and Beloborodov, Igor S.
 Description:
 We develop a theory of the exchange interaction between ferromagnetic (FM) metallic grains embedded into insulating matrix by taking into account the Coulomb blockade effects. For bulk ferromagnets separated by the insulating layer the exchange interaction strongly depends on the height and thickness of the tunneling barrier created by the insulator. We show that for FM grains embedded into insulating matrix the exchange coupling additionally depends on the dielectric properties of this matrix due to the Coulomb blockade effects. In particular, the FM coupling decreases with decreasing the dielectric permittivity of insulating matrix. We find that the change in the exchange interaction due to the Coulomb blockade effects can be a few tens of percent. Also, we study dependence of the intergrain exchange interaction on the grain size and other parameters of the system.
 Resource Type:
 Article
 Identifier:
 09538984
 Campus Tesim:
 Northridge
 Creator:
 Mahfouzi, Farzad and Kioussis, Nicholas G.
 Description:
 Motivated by the need to understand currentinduced magnetization dynamics at the nanoscale, we have developed a formalism, within the framework of Keldysh Green function approach, to study the currentinduced dynamics of a ferromagnetic (FM) nanoisland overlayer on a spinorbitcoupling (SOC) Rashba plane. In contrast to the commonly employed classical micromagnetic LLG simulations the magnetic moments of the FM are treated quantum mechanically. We obtain the density matrix of the whole system consisting of conduction electrons entangled with the local magnetic moments and calculate the effective damping rate of the FM. We investigate two opposite limiting regimes of FM dynamics: (1) The precessional regime where the magnetic anisotropy energy (MAE) and precessional frequency are smaller than the exchange interactions and (2) the local spinflip regime where the MAE and precessional frequency are comparable to the exchange interactions. In the former case, we show that due to the finite size of the FM domain, the “Gilbert damping” does not diverge in the ballistic electron transport regime, in sharp contrast to Kambersky's breathing Fermi surface theory for damping in metallic FMs. In the latter case, we show that above a critical bias the excited conduction electrons can switch the local spin moments resulting in demagnetization and reversal of the magnetization. Furthermore, our calculations show that the biasinduced antidamping efficiency in the local spinflip regime is much higher than that in the rotational excitation regime.
 Resource Type:
 Article
 Identifier:
 10980121
 Campus Tesim:
 Northridge
 Creator:
 Ancilotto, Francesco, Pi, Marti, Barranco, Manuel, and Eloranta, Jussi M.
 Description:
 Twodimensional flow past an infinitely long cylinder of nanoscopic radius in superfluid 4 He at zero temperature is studied using timedependent densityfunctional theory. The calculations reveal two distinct critical phenomena for the onset of dissipation: (i) vortexantivortex pair shedding from the periphery of the moving cylinder, and (ii) the appearance of cavitation in the wake, which possesses similar geometry to that observed experimentally for fastmoving micrometerscale particles in superfluid 4 He . The formation of cavitation bubbles behind the cylinder is accompanied by a sudden jump in the drag exerted on the moving cylinder by the fluid. Vortex pairs with the same circulation are occasionally emitted in the form of dimers, which constitute the building blocks for the Benard–von Karman vortex street structure observed in classical turbulent fluids and BoseEinstein condensates. The cavitationinduced dissipation mechanism should be common to all superfluids that are selfbound and have a finite surface tension, which include the recently discovered selfbound droplets in ultracold Bose gases. These systems would provide an ideal testing ground for further exploration of this mechanism experimentally.
 Resource Type:
 Article
 Identifier:
 10980121
 Campus Tesim:
 Northridge
 Creator:
 Ancilotto, Francesco, Pi, Marti, Hernando, Alberto, Halberstadt, Nadine, Coppens, Francois, Barranco, Manuel, Mateo, David, and Eloranta, Jussi M.
 Description:
 During the last decade, density function theory (DFT) in its static and dynamic time dependent forms, has emerged as a powerful tool to describe the structure and dynamics of doped liquid helium and droplets. In this review, we summarise the activity carried out in this field within the DFT framework since the publication of the previous review article on this subject [M. Barranco et al., J. Low Temp. Phys. 142, 1 (2006)]. Furthermore, a comprehensive presentation of the actual implementations of helium DFT is given, which have not been discussed in the individual articles or are scattered in the existing literature.
 Resource Type:
 Article
 Identifier:
 0144235X
 Campus Tesim:
 Northridge
 Creator:
 Grover, Tarum, Vishwanath, Ashvin, and Sheng, Donna
 Description:
 In contrast to ordinary symmetries, supersymmetry (SUSY) interchanges bosons and fermions. Originally proposed as a symmetry of our universe, it still awaits experimental verification. Here, we theoretically show that SUSY emerges naturally in condensed matter systems known as topological superconductors. We argue that the quantum phase transitions at the boundary of topological superconductors in both two and three dimensions display SUSY when probed at long distances and times. Experimental consequences include exact relations between quantities measured in disparate experiments and, in some cases, exact knowledge of the universal critical exponents. The topological surface states themselves may be interpreted as arising from spontaneously broken SUSY, indicating a deep relation between topological phases and SUSY.
 Resource Type:
 Article
 Identifier:
 00368075
 Campus Tesim:
 Northridge
 Creator:
 Ancilotto, Francesco, Pi, Marti, Hernando, Alberto, Halberstadt, Nadine, Coppens, Francois, Barranco, Manuel, Mateo, David, and Eloranta, Jussi M.
 Description:
 During the last decade, density function theory (DFT) in its static and dynamic time dependent forms, has emerged as a powerful tool to describe the structure and dynamics of doped liquid helium and droplets. In this review, we summarise the activity carried out in this field within the DFT framework since the publication of the previous review article on this subject [M. Barranco et al., J. Low Temp. Phys. 142, 1 (2006)]. Furthermore, a comprehensive presentation of the actual implementations of helium DFT is given, which have not been discussed in the individual articles or are scattered in the existing literature.
 Resource Type:
 Article
 Identifier:
 0144235X
 Campus Tesim:
 Northridge