Pi-phases in balanced fermionic superfluids on spin-dependent optical lattices

We study a balanced two-component system of ultracold fermions in one dimension with attractive interactions and subject to a spin-dependent optical lattice potential of opposite sign for the two components. We find states with different types of modulated pairing order parameters which are conceptually similar to pi-phases discussed for superconductor-ferromagnet heterostructures. Increasing the lattice depth induces sharp transitions between states of different parity. While the origin of the order parameter oscillations is similar to the FFLO phase for paired states with spin imbalance, the current system is intrinsically stable to phase separation. We discuss experimental requirements for creating and probing these novel phases.Comment: 4.3 pages, 4 figures, published versio

BCS-BEC crossover in bilayers of cold fermionic polar molecules

We investigate the quantum and thermal phase diagram of fermionic polar molecules loaded in a bilayer trapping potential with perpendicular dipole moment. We use both a BCS-theory approach that is most reliable at weak coupling and a strong-coupling approach that considers the two-body bound dimer states with one molecule in each layer as the relevant degree of freedom. The system ground state is a Bose-Einstein condensate (BEC) of dimer bound states in the low-density limit and a paired superfluid (BCS) state in the high-density limit. At zero temperature, the intralayer repulsion is found to broaden the regime of BCS-BEC crossover and can potentially induce system collapse through the softening of roton excitations. The BCS theory and the strongly coupled dimer picture yield similar predictions for the parameters of the crossover regime. The Berezinskii-Kosterlitz-Thouless transition temperature of the dimer superfluid is also calculated. The crossover can be driven by many-body effects and is strongly affected by the intralayer interaction which was ignored in previous studies

Dielectric function and plasmons in graphene

The electromagnetic response of graphene, expressed by the dielectric function, and the spectrum of collective excitations are studied as a function of wave vector and frequency. Our calculation is based on the full band structure, calculated within the tight-binding approximation. As a result, we find plasmons whose dispersion is similar to that obtained in the single-valley approximation by Dirac fermions. In contrast to the latter, however, we find a stronger damping of the plasmon modes due to inter-band absorption. Our calculation also reveals effects due to deviations from the linear Dirac spectrum as we increase the Fermi energy, indicating an anisotropic behavior with respect to the wave vector of the external electromagnetic field

Dynamical polarization, screening, and plasmons in gapped graphene

The one-loop polarization function of graphene has been calculated at zero temperature for arbitrary wavevector, frequency, chemical potential (doping), and band gap. The result is expressed in terms of elementary functions and is used to find the dispersion of the plasmon mode and the static screening within the random phase approximation. At long wavelengths the usual square root behaviour of plasmon spectra for two-dimensional (2D) systems is obtained. The presence of a small (compared to a chemical potential) gap leads to the appearance of a new undamped plasmon mode. At greater values of the gap this mode merges with the long-wavelength one, and vanishes when the Fermi level enters the gap. The screening of charged impurities at large distances differs from that in gapless graphene by slower decay of Friedel oscillations ($1/r^2$ instead of $1/r^3$), similarly to conventional 2D systems.Comment: 8 pages, 8 figures, v2: to match published versio

Dielectric function of the semiconductor hole gas

We study the dielectric function of the homogeneous hole gas in p-doped zinc-blende III-V bulk semiconductors within random phase approximation with the valence band being modeled by Luttinger's Hamiltonian in the spherical approximation. In the static limit we find a beating of Friedel oscillations between the two Fermi momenta for heavy and light holes, while at large frequencies dramatic corrections to the plasmon dispersion occur.Comment: 4 pages, 1 figure included. Version to appear in Europhys. Let

Electron-electron interaction and charging effects in graphene quantum dots

We analyze charging effects in graphene quantum dots. Using a simple model, we show that, when the Fermi level is far from the neutrality point, charging effects lead to a shift in the electrostatic potential and the dot shows standard Coulomb blockade features. Near the neutrality point, surface states are partially occupied and the Coulomb interaction leads to a strongly correlated ground state which can be approximated by either a Wigner crystal or a Laughlin like wave function. The existence of strong correlations modify the transport properties which show non equilibrium effects, similar to those predicted for tunneling into other strongly correlated systems.Comment: Extended version accepted for publication at Phys. Rev.

Double Quantum Dots in Carbon Nanotubes

We study the two-electron eigenspectrum of a carbon-nanotube double quantum dot with spin-orbit coupling. Exact calculation are combined with a simple model to provide an intuitive and accurate description of single-particle and interaction effects. For symmetric dots and weak magnetic fields, the two-electron ground state is antisymmetric in the spin-valley degree of freedom and is not a pure spin-singlet state. When double occupation of one dot is favored by increasing the detuning between the dots, the Coulomb interaction causes strong correlation effects realized by higher orbital-level mixing. Changes in the double-dot configuration affect the relative strength of the electron-electron interactions and can lead to different ground state transitions. In particular, they can favor a ferromagnetic ground state both in spin and valley degrees of freedom. The strong suppression of the energy gap can cause the disappearance of the Pauli blockade in transport experiments and thereby can also limit the stability of spin-qubits in quantum information proposals. Our analysis is generalized to an array of coupled dots which is expected to exhibit rich many-body behavior.Comment: 14 pages, 11 pages and 1 table. Typos in text and Figs.4 and 6 correcte

Casimir interactions in graphene systems

The non-retarded Casimir interaction (van der Waals interaction) between two free standing graphene sheets as well as between a graphene sheet and a substrate is determined. An exact analytical expression is given for the dielectric function of graphene along the imaginary frequency axis within the random phase approximation for arbitrary frequency, wave vector, and doping.Comment: 4 pages, 4 figure

Zetos: Ein Kulturbelastungssystem für trabekuläre Knochen. Untersuchung eines Belastungssignals mit verschiedenen Intensitäten an bovinen Knochenzylindern

Das Ziel dieses Versuches war es, den Einfluss verschiedener Intensitäten (1000, 1500, 2000, 3000, 4000µstrain) eines physiologischen Belastungssignals (Springen) auf trabekulären Knochen zu untersuchen. Dazu wurde der Zetos: Ein Kultur- und Belastungssystem für trabekulären Knochen verwendet. 48 bovine Knochenexplantate wurden randomisiert und in 6 Gruppen (je 8) eingeteilt. 5 Belastungsgruppen und eine Kontrollgruppe. Die Knochenexplantate wurden für 26 Tage kultiviert (DMEM HighGlucose Medium) und an 23 Tagen mechanisch belastet. Vor jeder Belastung wurde täglich (belastete Gruppen) und alle 3 Tage (Kontrollgruppe) die Steifigkeit (Young's Modulus) gemessen. Bei Auswertung der Steifigkeitsmessung gab es signifikante Unterschiede (p<0,05) zwischen der Kontrollgruppe und den Belastungsgruppen ab 1500µstrain Maximalbelastung. In den Gruppen mit 1500, 2000, 3000µstrain konnte ein kontinuierlicher Steifigkeitsanstieg über den gesamten Versuchszeitraum festgestellt werden. Die Gruppe mit 4000µstrain erreichte schon nach sehr kurzer Zeit (8.Belastungstag) ein Steifigkeitsmaximum. Dies blieb bis zum Versuchende konstant. Die gemessenen Osteoiddicken in dieser Gruppe waren aber nicht größer als in den Gruppen mit 2000 und 3000µstrain. Die Gruppe mit 2000µstrain zeigte den höchsten Anteil neu gebildeten Osteoids. Diese 3 genannten Gruppen unterschieden sich aber signifikant (p<0,05) von der Kontrollgruppe. Zudem konnte eine Korrelation zwischen ansteigender Intensität und Osteoidzunahme beobachtet werden (r = 0,87). Die histologische Auswertung erfolgte an nicht entkalktem Knochen und zeigte gut erhaltene trabekuläre Strukturen sowie ebenso gut erhaltene Zellmorphologien