Violation of the entropic area law for Fermions

We investigate the scaling of the entanglement entropy in an infinite translational invariant Fermionic system of any spatial dimension. The states under consideration are ground states and excitations of tight-binding Hamiltonians with arbitrary interactions. We show that the entropy of a finite region typically scales with the area of the surface times a logarithmic correction. Thus, in contrast to analogous Bosonic systems, the entropic area law is violated for Fermions. The relation between the entanglement entropy and the structure of the Fermi surface is discussed, and it is proven, that the presented scaling law holds whenever the Fermi surface is finite. This is in particular true for all ground states of Hamiltonians with finite range interactions.Comment: 5 pages, 1 figur

Dynamical Coulomb Blockade Observed in Nano-Sized Electrical Contacts

Electrical contacts between nano-engineered systems are expected to constitute the basic building blocks of future nano-scale electronics. However, the accurate characterization and understanding of electrical contacts at the nano-scale is an experimentally challenging task. Here we employ low-temperature scanning tunneling spectroscopy to investigate the conductance of individual nano-contacts formed between flat Pb islands and their supporting substrates. We observe a suppression of the differential tunnel conductance at small bias voltages due to dynamical Coulomb blockade effects. The differential conductance spectra allow us to determine the capacitances and resistances of the electrical contacts which depend systematically on the island--substrate contact area. Calculations based on the theory of environmentally assisted tunneling agree well with the measurements.Comment: 5 pages, 3 figures, to appear in PR

Spin Coulomb drag in the two-dimensional electron liquid

We calculate the spin-drag transresistivity $\rho_{\uparrow \downarrow}(T)$ in a two-dimensional electron gas at temperature $T$ in the random phase approximation. In the low-temperature regime we show that, at variance with the three-dimensional low-temperature result [$\rho_{\uparrow\downarrow}(T) \sim T^2$], the spin transresistivity of a two-dimensional {\it spin unpolarized} electron gas has the form $\rho_{\uparrow\downarrow}(T) \sim T^2 \ln T$. In the spin-polarized case the familiar form $\rho_{\uparrow\downarrow}(T) =A T^2$ is recovered, but the constant of proportionality $A$ diverges logarithmically as the spin-polarization tends to zero. In the high-temperature regime we obtain $\rho_{\uparrow \downarrow}(T) = -(\hbar / e^2) (\pi^2 Ry^* /k_B T)$ (where $Ry^*$ is the effective Rydberg energy) {\it independent} of the density. Again, this differs from the three-dimensional result, which has a logarithmic dependence on the density. Two important differences between the spin-drag transresistivity and the ordinary Coulomb drag transresistivity are pointed out: (i) The $\ln T$ singularity at low temperature is smaller, in the Coulomb drag case, by a factor $e^{-4 k_Fd}$ where $k_F$ is the Fermi wave vector and $d$ is the separation between the layers. (ii) The collective mode contribution to the spin-drag transresistivity is negligible at all temperatures. Moreover the spin drag effect is, for comparable parameters, larger than the ordinary Coulomb drag effect.Comment: 6 figures; various changes; version accepted for publicatio

Sequential Generation of Matrix-Product States in Cavity QED

We study the sequential generation of entangled photonic and atomic multi-qubit states in the realm of cavity QED. We extend the work of C. Schoen et al. [Phys. Rev. Lett. 95, 110503 (2005)], where it was shown that all states generated in a sequential manner can be classified efficiently in terms of matrix-product states. In particular, we consider two scenarios: photonic multi-qubit states sequentially generated at the cavity output of a single-photon source and atomic multi-qubit states generated by their sequential interaction with the same cavity mode.Comment: 11 page

Pressure Effect and Specific Heat of RBa2Cu3Ox at Distinct Charge Carrier Concentrations: Possible Influence of Stripes

In YBa2Cu3Ox, distinct features are found in the pressure dependence of the transition temperature, dTc/dp, and in DeltaCp*Tc, where DeltaCp is the jump in the specific heat at Tc: dTc/dp becomes zero when DeltaCp*Tc is maximal, whereas dTc/dp has a peak at lower oxygen contents where DeltaCp*Tc vanishes. Substituting Nd for Y and doping with Ca leads to a shift of these specific oxygen contents, since oxygen order and hole doping by Ca influences the hole content nh in the CuO2 planes. Calculating nh from the parabolic Tc(nh) behavior, the features coalesce for all samples at nh=0.11 and nh=0.175, irrespective of substitution and doping. Hence, this behavior seems to reflect an intrinsic property of the CuO2 planes. Analyzing our results we obtain different mechanisms in three doping regions: Tc changes in the optimally doped and overdoped region are mainly caused by charge transfer. In the slightly underdoped region an increasing contribution to dTc/dp is obtained when well ordered CuO chain fragments serve as pinning centers for stripes. This behavior is supported by our results on Zn doped NdBa2Cu3Ox and is responsible for the well known dTc/dp peak observed in YBa2Cu3Ox at x=6.7. Going to a hole content below nh=0.11 our results point to a crossover from an underdoped superconductor to a doped antiferromagnet, changing completely the physics of these materials.Comment: 6 pages, 5 figures Proccedings of the 'Stripes 2000' Conference, Rome (2000

High expression of cannabinoid receptor 2 on cytokine-induced killer cells and multiple myeloma cells

Multiple myeloma (MM) is characterized by aberrant bone marrow plasma cell (PC) proliferation and is one of the most common hematological malignancies. The potential effect of cannabinoids on the immune system and hematological malignancies has been poorly characterized. Cannabidiol (CBD) may be used to treat various diseases. CBD is known to exert immunomodulatory effects through the activation of cannabinoid receptor 2 (CB2), which is expressed in high levels in the hematopoietic system. Cytokine-induced killer (CIK) cells are a heterogeneous population of polyclonal T lymphocytes obtained via ex vivo sequential incubation of peripheral blood mononuclear cells (PBMCs) with interferon-γ (IFN-γ), anti CD3 monoclonal antibody, and IL-2. They are characterized by the expression of CD3+ and CD56+, which are surface markers common to T lymphocytes and natural killer (NK) cells. CIK cells are mainly used in hematological patients who suffer relapse after allogeneic transplantation. Here, we investigated their antitumor effect in combination with pure cannabidiol in KMS-12 MM cells by lactate dehydrogenase LDH cytotoxicity assay, CCK-8 assay, and flow cytometry analysis. The surface and intracellular CB2 expressions on CIK cells and on KMS-12 and U-266 MM cell lines were also detected by flow cytometry. Our findings confirm that the CB2 receptor is highly expressed on CIK cells as well as on MM cells. CBD was able to decrease the viability of tumor cells and can have a protective role for CIK cells. It also inhibits the cytotoxic activity of CIKs against MM at high concentrations, so in view of a clinical perspective, it has to be considered that the lower concentration of 1 µM can be used in combination with CIK cells. Further studies will be required to address the mechanism of CBD modulation of CIK cells in more detail

Air hydrodynamics of the ultrafast laser-triggered spark gap

We present space and time resolved measurements of the air hydrodynamics induced by ultrafast laser pulse excitation of the air gap between two electrodes at high potential difference. We explore both plasma-based and plasma-free gap excitation. The former uses the plasma left in the wake of femtosecond filamentation, while the latter exploits air heating by multiple-pulse resonant excitation of quantum molecular wavepackets. We find that the cumulative electrode-driven air density depression channel initiated by the laser plays the dominant role in the gap evolution leading to breakdown

Lattice Expansion in Seamless Bi layer Graphene Constrictions at High Bias

Our understanding of sp2 carbon nanostructures is still emerging and is important for the development of high performance all carbon devices. For example, in terms of the structural behavior of graphene or bi-layer graphene at high bias, little to nothing is known. To this end we investigated bi-layer graphene constrictions with closed edges (seamless) at high bias using in situ atomic resolution transmission electron microscopy. We directly observe a highly localized anomalously large lattice expansion inside the constriction. Both the current density and lattice expansion increase as the bi-layer graphene constriction narrows. As the constriction width decreases below 10 nm, shortly before failure, the current density rises to 4 \cdot 109 A cm-2 and the constriction exhibits a lattice expansion with a uniaxial component showing an expansion approaching 5 % and an isotropic component showing an expansion exceeding 1 %. The origin of the lattice expansion is hard to fully ascribe to thermal expansion. Impact ionization is a process in which charge carriers transfer from bonding states to antibonding states thus weakening bonds. The altered character of C-C bonds by impact ionization could explain the anomalously large lattice expansion we observe in seamless bi-layer graphene constrictions. Moreover, impact ionization might also contribute to the observed anisotropy in the lattice expansion, although strain is probably the predominant factor.Comment: to appear in NanoLetter

Collective mechanism of dilepton production in high-energy nuclear collisions

Collective bremsstrahlung of vector meson fields in relativistic nuclear collisions is studied within the time-dependent Walecka model. Mutual deceleration of the colliding nuclei is described by introducing the effective stopping time and average rapidity loss of baryons. It is shown that electromagnetic decays of virtual omega-mesons produced by bremsstrahlung mechanism can provide a substantial contribution to the soft dilepton yield at the SPS bombarding energies. In particular, it may be responsible for the dilepton enhancement observed in 160 AGev central Pb+Au collisions. Suggestions for future experiments to estimate the relative contribution of the collective mechanism are given.Comment: 6 page

Spin-filter tunnel junction with matched Fermi surfaces

Efficient injection of spin-polarized current into a semiconductor is a basic prerequisite for building semiconductor-based spintronic devices. Here, we use inelastic electron tunneling spectroscopy to show that the efficiency of spin-filter-type spin injectors is limited by spin scattering of the tunneling electrons. By matching the Fermi-surface shapes of the current injection source and target electrode material, spin injection efficiency can be significantly increased in epitaxial ferromagnetic insulator tunnel junctions. Our results demonstrate that not only structural but also Fermi-surface matching is important to suppress scattering processes in spintronic devices.Comment: 5 pages, 4 figure