arXiv.org: Condensed Matter

cond-mat updates on arXiv.org

• Lattice Boltzmann simulations of liquid crystalline fluids: active gels and blue phases. (arXiv:1009.1153v1 [cond-mat.soft])
  

  Lattice Boltzmann simulations have become a method of choice to solve the hydrodynamic equations of motion of a number of complex fluids. Here we review some recent applications of lattice Boltzmann to study the hydrodynamics of liquid crystalline materials. In particular, we focus on the study of (a) the exotic blue phases of cholesteric liquid crystals, and (b) active gels - a model system for actin plus myosin solutions or bacterial suspensions. In both cases lattice Boltzmann studies have proved useful to provide new insights into these complex materials.

  
  
  
• Quantum dot Rabi rotations beyond the weak exciton-phonon coupling regime. (arXiv:1009.1155v1 [cond-mat.mes-hall])
  

  We study the excitonic dynamics of a driven quantum dot under the influence of a phonon environment, going beyond the weak exciton-phonon coupling approximation. By combining the polaron transform and time-local projection operator techniques we develop a master equation that can be valid over a much larger range of exciton-phonon coupling strengths and temperatures than the standard weak-coupling approach. For the experimentally relevant parameters considered here, we find that the weak-coupling and polaron theories give very similar predictions for low temperatures (below 30 K), while at higher temperatures we begin to see discrepancies between the two. This is due to the fact that, unlike the polaron approach, the weak-coupling theory is incapable of capturing multiphonon effects, while it also does not properly account for phonon-induced renormalisation of the driving frequency. In particular, we find that the weak-coupling theory often overestimates the damping rate when compared to that predicted by the polaron theory. Finally, we extend our theory to include non-Markovian effects and find that, for the parameters considered here, they have little bearing on the excitonic Rabi rotations when plotted as a function of pulse area.

  
  
  
• A finite-temperature liquid-quasicrystal transition in a lattice model. (arXiv:1009.1156v1 [cond-mat.stat-mech])
  

  We consider a tiling model of the two-dimensional square-lattice, where each site is tiled with one of the sixteen Wang tiles. The ground states of this model are all quasi-periodic. The systems undergoes a disorder to quasi-periodicity phase transition at finite temperature. Introducing a proper order-parameter, we study the system at criticality, and extract the critical exponents characterizing the transition. The exponents obtained are consistent with hyper-scaling.

  
  
  
• Effects of vertex corrections on diagrammatic approximations applied to the study of transport through a quantum dot. (arXiv:1009.1157v1 [cond-mat.str-el])
  

  In the present work, we calculate the conductance through a single quantum dot weakly coupled to metallic contacts. We use the spin-1/2 Anderson model to describe the quantum dot, while considering a finite Coulomb repulsion. We solve the interacting system using the non-crossing-approximation (NCA) and the one-crossing approximation (OCA). We obtain the linear response conductance as a function of temperature and energy position of the localized level. From the comparison of both approximations we extract the role of the vertex corrections, which are introduced in the OCA calculations and neglected in the NCA scheme. As a function of the energy position, we observe that the diagrams omitted within NCA are really important for appropriately describing transport phenomena in Kondo systems as well as in the mixed valence regime. On the other hand, as a function of temperature, the corrections introduced by OCA partly recover the universal scaling properties known from numerical approaches such as the Numerical Renormalization Group(NRG).

  
  
  
• Nanoelectromechanical coupling in fullerene peapods probed via resonant electrical transport experiments. (arXiv:1009.1168v1 [cond-mat.mes-hall])
  

  Fullerene peapods, that is carbon nanotubes encapsulating fullerene molecules, can offer enhanced functionality with respect to empty nanotubes. However, the present incomplete understanding of how a nanotube is affected by entrapped fullerenes is an obstacle for peapods to reach their full potential in nanoscale electronic applications. Here, we investigate the effect of C60 fullerenes on electron transport via peapod quantum dots. Compared to empty nanotubes, we find an abnormal temperature dependence of Coulomb blockade oscillations, indicating the presence of a nanoelectromechanical coupling between electronic states of the nanotube and mechanical vibrations of the fullerenes. This provides a method to detect the C60 presence and to probe the interplay between electrical and mechanical excitations in peapods, which thus emerge as a new class of nanoelectromechanical systems.

  
  
  
• Spin Bose-Metal and Valence Bond Solid phases in a spin-1/2 model with ring exchanges on a four-leg triangular ladder. (arXiv:1009.1179v1 [cond-mat.str-el])
  

  We study a spin-1/2 system with Heisenberg plus ring exchanges on a four-leg triangular ladder using Density Matrix Renormalization Group and Gutzwiller variational wavefunctions. Near an isotropic lattice regime, for moderate to large ring exchanges we find a Spin Bose-Metal phase with spinon Fermi sea consisting of three partially filled bands. Going away from the triangular towards square lattice regime, we find a staggered dimer phase with dimers in the transverse direction, while for small ring exchanges the system is in a featureless rung phase. We also discuss parent states and a possible phase diagram in two dimensions.

  
  
  
• Exact Results on Potts Model Partition Functions in a Generalized External Field and Weighted-Set Graph Colorings. (arXiv:1009.1182v1 [cond-mat.stat-mech])
  

  We present exact results on the partition function of the $q$-state Potts model on various families of graphs $G$ in a generalized external magnetic field that favors or disfavors spin values in a subset $I_s = \{1,...,s\}$ of the total set of possible spin values, $Z(G,q,s,v,w)$, where $v$ and $w$ are temperature- and field-dependent Boltzmann variables. We remark on differences in thermodynamic behavior between our model with a generalized external magnetic field and the Potts model with a conventional magnetic field that favors or disfavors a single spin value. Exact results are also given for the interesting special case of the zero-temperature Potts antiferromagnet, corresponding to a set-weighted chromatic polynomial $Ph(G,q,s,w)$ that counts the number of colorings of the vertices of $G$ subject to the condition that colors of adjacent vertices are different, with a weighting $w$ that favors or disfavors colors in the interval $I_s$. We derive powerful new upper and lower bounds on $Z(G,q,s,v,w)$ for the ferromagnetic case in terms of zero-field Potts partition functions with certain transformed arguments. We also prove general inequalities for $Z(G,q,s,v,w)$ on different families of tree graphs. As part of our analysis, we elucidate how the field-dependent Potts partition function and weighted-set chromatic polynomial distinguish, respectively, between Tutte-equivalent and chromatically equivalent pairs of graphs.

  
  
  
• Electron beam formation from spin-orbit interactions in zincblende semiconductor quantum wells. (arXiv:1009.1190v1 [cond-mat.mes-hall])
  

  We find a dramatic enhancement of electron propagation along a narrow range of real-space angles from an isotropic source in a two-dimensional quantum well made from a zincblende semiconductor. This ``electron beam'' formation is caused by the interplay between spin-orbit interaction originating from a perpendicular electric field to the quantum well and the intrinsic spin-orbit field of the zincblende crystal lattice in a quantum well, in situations where the two fields are different in strength but of the same order of magnitude. Beam formation is associated with caustics and can be described semi-classically using a stationary phase analysis.

  
  
  
• Universality of Cluster Dynamics. (arXiv:1009.1191v1 [cond-mat.stat-mech])
  

  We have studied the kinetics of cluster formation for dynamical systems of dimensions up to $n=8$ interacting through elastic collisions or coalescence. These systems could serve as possible models for gas kinetics, polymerization and self-assembly. In the case of elastic collisions, we found that the cluster size probability distribution undergoes a phase transition at a critical time which can be predicted from the average time between collisions. This enables forecasting of rare events based on limited statistical sampling of the collision dynamics over short time windows. The analysis was extended to L$^p$-normed spaces ($p=1,\dots,\infty$) to allow for some amount of interpenetration or volume exclusion. The results for the elastic collisions are consistent with previously published low-dimensional results in that a power law is observed for the empirical cluster size distribution at the critical time. We found that the same power law also exists for all dimensions $n=2,\dots,8$, 2D L$^p$ norms, and even for coalescing collisions in 2D. This broad universality in behavior may be indicative of a more fundamental process governing the growth of clusters.

  
  
  
• Hopf Maps, Lowest Landau Level, and Fuzzy Spheres. (arXiv:1009.1192v1 [hep-th])
  

  This paper is a review of monopoles, lowest Landau level, fuzzy spheres, and their mutual relations. The Hopf maps of division algebras provide a prototype relation between monopoles and fuzzy spheres. Generalization of complex numbers to Clifford algebra is exactly analogous to generalization of fuzzy two-spheres to higher dimensional fuzzy spheres. Higher dimensional fuzzy spheres have an interesting hierarchical structure made of ''compounds'' of lower dimensional spheres. We give a physical interpretation for such particular structure of fuzzy spheres by utilizing Landau models in generic even dimensions. With Grassmann algebra, we also introduce a graded version of the Hopf map, and discuss its relation to fuzzy supersphere in context of supersymmetric Landau model.

  
  
  
• Composite-Fermion Theory for Pseudogap, Fermi Arc, Hole Pocket, and Non-Fermi-Liquid of Underdoped Cuprate Superconductors. (arXiv:1009.1197v1 [cond-mat.str-el])
  

  We propose that an extension of the exciton concept to doped Mott insulators offers a fruitful insight into challenging issues of the copper oxide superconductors. In our extension, new fermionic excitations called cofermions emerge in conjunction to generalized excitons. The cofermions hybridize with conventional quasiparticles. Then a hybridization gap opens, and is identified as the pseudogap observed in the underdoped cuprates. The resultant Fermi-surface reconstruction naturally explains a number of unusual properties of the underdoped cuprates, such as the Fermi arc and/or pocket formation.

  
  
  
• Quantized Anomalous Hall Insulator in a Nanopatterned Two-Dimensional Electron Gas. (arXiv:1009.1200v1 [cond-mat.mtrl-sci])
  

  We propose that a quantum anomalous Hall insulator (QAHI) can be realized in a nanopatterned two-dimensional electron gas (2DEG) with an in-plane magnetic field. The Berry curvatures originating from the in-plane magnetic field and Rashba and Dresselhaus spin-orbit coupling, in combination with a nanoscale honeycomb lattice potential modulation, lead to topologically nontrivial insulating states in the 2DEG. In the bulk insulating gaps, the anomalous Hall conductivity is quantized $-e^{2}/h$, corresponding to a finite Chern number $-1$. There exists one gapless chiral edge state on each edge of a finite size 2DEG.

  
  
  
• Polarization Dependence of Raman Spectra in Strained Graphene. (arXiv:1009.1209v1 [cond-mat.mes-hall])
  

  The polarization dependences of the G, D, and 2D (G$'$) bands in Raman spectra at graphene bulk and edge are examined theoretically. The 2D and D bands have different selection rules at bulk and edge. At bulk, the 2D band intensity is maximum when the polarization of the scattered light is parallel to that of incident light, whereas the D band intensity does not have a polarization dependence. At edge, the 2D and D bands exhibit a selection rule similar to that of the G band proposed in a previous paper. We suggest that a constraint equation on the axial velocity caused by the graphene edge is essential for the dependence of the G band on the crystallographic orientation observed in the bulk of strained graphene. This is indicative of that the pseudospin and valleyspin in the bulk of graphene can not be completely free from the effect of surrounding edge. The status of the experiments on the G and D bands at the graphene edge is mentioned.

  
  
  
• Direct and Inverse Magnetocaloric effects in A-site ordered PrBaMn2O6 manganite in low magnetic fields. (arXiv:1009.1228v1 [cond-mat.str-el])
  

  The magnetocaloric effect (MCE) of A-site ordered PrBaMn2O6 manganite has been studied by direct methods and by the specific heat measurements. Direct measurements of the MCE in low magnetic fields were performed using recently proposed modulation technique and by classic direct method in high fields. Direct and inverse MCE are observed at Curie and Neel points correspondingly. A value of the inverse MCE in the heating run is less than in the cooling regime. This effect can be attributing to competition between ferromagnetic and antiferromagnetic interactions. Indirectly estimated and direct MCE values considerably differ in around first order AF transition.

  
  
  
• First-principles calculations of graphene nanoribbons in gaseous environments: Structural and electronic properties. (arXiv:1009.1242v1 [cond-mat.mes-hall])
  

  The stability of graphene nanoribbons in the presence of typical atmospheric molecules is systematically investigated by means of density functional theory. We calculate the edge formation free energy of five different edge configurations passivated by H, H$_2$, O, O$_2$, N$_2$, CO, CO$_2$, and H$_2$O, respectively. In addition to the well known hydrogen passivated armchair and zig-zag edges, we find the zig-zag edge saturated by oxygen atoms to be particularly stable under atmospheric conditions. Saturation by oxygen leads to the formation of metallic states strictly localized on the oxygen atoms. Finally, the vibrational spectrum of the hydrogen and oxygen passivated ribbons are calculated and compared.

  
  
  
• Symmetries in Fluctuations Far from Equilibrium. (arXiv:1009.1243v1 [cond-mat.stat-mech])
  

  Fluctuations arise universally in Nature as a reflection of the discrete microscopic world at the macroscopic level. Despite their apparent noisy origin, fluctuations encode fundamental aspects of the physics of the system at hand, crucial to understand irreversibility and nonequilibrium behavior. In order to sustain a given fluctuation, a system traverses a precise optimal path in phase space. Here we show that by demanding invariance of optimal paths under symmetry transformations, new and general fluctuation relations valid arbitrarily far from equilibrium are unveiled. This opens an unexplored route toward a deeper understanding of nonequilibrium physics by bringing symmetry principles to the realm of fluctuations. We illustrate this concept studying symmetries of the current distribution out of equilibrium. In particular we derive an isometric fluctuation relation which links in a strikingly simple manner the probabilities of any pair of isometric current fluctuations. This relation, which results from the time-reversibility of the dynamics, includes as a particular instance the Gallavotti-Cohen fluctuation theorem in this context but adds a completely new perspective on the high level of symmetry imposed by time-reversibility on the statistics of nonequilibrium fluctuations. We confirm the validity of the new symmetry relation in extensive numerical simulations, and suggest that the idea of symmetry in fluctuations as invariance of optimal paths has far-reaching consequences in diverse fields.

  
  
  
• Subgap conductance in superconducting tunnel junctions. (arXiv:1009.1248v1 [cond-mat.supr-con])
  

  In highly resistive superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions, "excess" subgap current is usually observed. We have studied subgap conductance, G_sg, in Al/AlOx/Al and Al/AlOx/Cu tunnel junctions. In the former, we observed a large decrease of G_sg in the transition from the SIS to the SIN regime. In the latter, we observed several signatures of coherent diffusive two-particle transport. We use the quasiclassical Keldysh-Green function theory to quantify the contributions of the single- and two-particle processes on G_sg. We argue that our observations rule out the common ``pinhole" scenario for the origin of the excess subgap current. We also find that the residual dissipation associated with the single-particle current is of the correct magnitude to limit the relaxation time in state-of-the-art qubit measurements.

  
  
  
• Ewald methods for inverse power-law interactions in tridimensional and quasi-two dimensional systems. (arXiv:1009.1255v1 [cond-mat.stat-mech])
  

  In this paper, we derive the Ewald method for inverse power-law interactions in quasi-two dimensional systems. The derivation is done by using two different analytical methods. The first uses the Parry's limit, that considers the Ewald methods for quasi-two dimensional systems as a limit of the Ewald methods for tridimensional systems, the second uses Poisson-Jacobi identities for lattice sums. Taking into account the equivalence of both derivations, we obtain a new analytical Fourier transform intregral involving incomplete gamma function. Energies of the generalized restrictive primitive model of electrolytes ($\eta$-RPM) and of the generalized one component plasma model ($\eta$-OCP) are given for the tridimensional, quasi-two dimensional and monolayers systems. Few numerical results, using Monte-Carlo simulations, for $\eta$-RPM and $\eta$-OCP monolayers systems are reported.

  
  
  
• Can we predict the failure point of a loaded composite material?. (arXiv:1009.1264v1 [cond-mat.mtrl-sci])
  

  As a model of composite material, the fiber bundle model has been chosen -where a bundle of fibers is subjected to external load and fibers have distributed thresholds. For different loading conditions, such a system shows few precursors which indicate that the complete failure is imminent. When external load is increased quasi-statically - \textit{bursts} (number of failing fibers) of different sizes are produced. The burst statistics shows a robust crossover behavior near the failure point, around which the average burst size seems to diverge. If the load is increased by discrete steps, susceptibility and relaxation time diverge as failure point is approached. When the bundle is overloaded (external load is more than critical load) the rate of breaking shows a minimum at half way to the collapse point. The pattern and statistics of energy emission bursts show characteristic difference for below-critical and over-critical load levels.

  
  
  
• Prediction of the collapse point of overloaded materials by monitoring energy emissions. (arXiv:1009.1268v1 [cond-mat.mtrl-sci])
  

  A bundle of many fibers with stochastically distributed breaking thresholds is considered as a model of composite materials. The fibers are assumed to share the load equally, and to obey Hookean elasticity up to the breaking point. The bundle is slightly overloaded, which leads to complete failure. We study the properties of emission bursts in which an amount of energy $E$ is released. The analysis shows that the size of the energy bursts has a minimum when the system is half-way from the collapse point.

  
  
  
• Thermodynamic Spin Glass Phase Induced by Weak Random Exchange Disorder in a Classical Spin Liquid: the Case of the Pyrochlore Heisenberg Antiferromagnet. (arXiv:1009.1272v1 [cond-mat.stat-mech])
  

  The glassy behavior observed in the pyrochlore magnet Y2Mo2O7, where the magnetic Mo^{4+} ions interact predominantly via isotropic nearest neighbor antiferromagnetic exchange, possibly with additional weak disorder, is a distinct class of spin glass systems where frustration is mostly geometrical. A model proposed to describe such a spin glass behavior is the Heisenberg model on a pyrochlore lattice with random but strictly antiferromagnetic exchange disorder. In this paper, we provide compelling numerical evidence from extensive Monte Carlo simulations which show that the model exhibits a finite temperature spin glass transition and thus is a realization of a spin glass induced by random weak disorder from spin liquid. From our results, we are led to suggest that the spin glass state of Y2Mo2O7 is driven by effective strong disorder.

  
  
  
• Phase coherent transport in SrTiO3/LaAlO3 interfaces. (arXiv:1009.1273v1 [cond-mat.mes-hall])
  

  The two dimensional electron gas formed between the two band insulators SrTiO3 and LaAlO3 exhibits a variety of interesting physical properties which make it an appealing material for use in future spintronics and/or quantum computing devices. For this kind of applications electrons have to retain their phase memory for sufficiently long times or length. Using a mesoscopic size device we were able to extract the phase coherence length, and its temperature variation. We find the dephasing rate to have a power law dependence on temperature. The power depends on the temperature range studied and sheet resistance as expected from dephasing due to strong electron-electron interactions.

  
  
  
• Three dimensional instability of flexible ferromagnetic filament loop. (arXiv:1009.1276v1 [cond-mat.soft])
  

  Dynamics of flexible ferromagnetic filaments in an external magnetic field is considered. We report the existence of a buckling instability of the ferromagnetic filament at the magnetic field reversion, which leads to the formation of a metastable loop. Its relaxation through three dimensional transformation of the configurations is observed experimentally and confirmed by numerical simulations. Bending modulus of the flexible ferromagnetic filaments synthesized by linking micron size core-shell ferromagnetic particles with DNA fragments is estimated by comparison of the parameters of the loops observed in the experiment with theoretical calculations. Formation of the loop and its relaxation are characterized by the numerically calculated writhe number. The relaxation time of the loop allows us to estimate the hydrodynamic drag of the filament.

  
  
  
• Preparation of Highly Crystalline TiO2 Nanostructures by Acid-assisted Hydrothermal Treatment of Hexagonal-structured Nanocrystalline Titania/Cetyltrimethyammonium Bromide Nanoskeleton. (arXiv:1009.1283v1 [cond-mat.mtrl-sci])
  

  Highly crystalline TiO2 nanostructures were prepared through a facile inorganic acid-assisted hydrothermal treatment of hexagonal-structured assemblies of nanocrystalline titiania templated by cetyltrimethylammonium bromide (Hex-ncTiO2/CTAB Nanoskeleton) as starting materials. All samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The influence of hydrochloric acid concentration on the morphology, crystalline and the formation of the nanostructures were investigated. We found that the morphology and crystalline phase strongly depended on the hydrochloric acid concentrations. More importantly, crystalline phase was closely related to the morphology of TiO2 nanostructure. Nanoparticles were polycrystalline anatase phase, and aligned nanorods were single crystalline rutile phase. Possible formation mechanisms of TiO2 nanostructures with various crystalline phases and morphologies were proposed.

  
  
  
• Electron-induced rippling in graphene. (arXiv:1009.1285v1 [cond-mat.mes-hall])
  

  We show that the interaction between flexural phonons, when corrected by the exchange of electron-hole excitations, may place the graphene sheet very close to a quantum critical point characterized by the strong suppression of the bending rigidity of the membrane. Ripples arise then due to spontaneous symmetry breaking, following a mechanism similar to that responsible for the condensation of the Higgs field in relativistic field theories. In the presence of membrane tensions, ripple condensation may be reinforced or suppressed depending on the sign of the tension, following a zero-temperature buckling transition in which the order parameter is given essentially by the square of the gradient of the flexural phonon field.

  
  
  
• Mapping of functionalized regions on carbon nanotubes by scanning tunneling microscopy. (arXiv:1009.1290v1 [cond-mat.mes-hall])
  

  Scanning tunneling microscopy (STM) gives us the opportunity to map the surface of functionalized carbon nanotubes in an energy resolved manner and with atomic precision. But this potential is largely untapped, mainly due to sample stability issues which inhibit reliable measurements. Here we present a simple and straightforward solution that makes away with this difficulty, by incorporating the functionalized multiwalled carbon nanotubes (MWCNT) into a few layer graphene - nanotube composite. This enabled us to measure energy resolved tunneling conductance maps on the nanotubes, which shed light on the level of doping, charge transfer between tube and functional groups and the dependence of defect creation or functionalization on crystallographic orientation.

  
  
  
• Diffusion-annihilation proecesses in weighted scale-free networks with identical degree sequence. (arXiv:1009.1294v1 [cond-mat.dis-nn])
  

  The studies based on $A+A\rightarrow\emptyset$ and $A+B\rightarrow\emptyset$ diffusion-annihilation processes have so far been studied on weighted networks, e.g, small-world networks and scale-free networks. The influences of $\theta$ to the processes are apparent on scale-free networks but weak on the the small-world ones. The reason of this phenomenon is ascribed to the hubs, which attract random walkers to visit them and react with each other. At the same time, the large weight of links can strength the attraction. However this conclusion is questioned on fractal scale-free networks. Here we examine the diffusion-annihilation processes on a family of one-parameter (denoted by $q$), which can present both fractal and unfractal properties for different $q$. We observe that the weight of links doesn't enhance the efficiency of reaction on the fractal scale-free networks as on their unfractal counterparts, conversely, it delays the process a bit. This highlights the key role that fractal topological properties plays in diffusion-annihilation process and the necessity to study it on weighted fractal networks.

  
  
  
• Efimov physics in bosonic atom-trimer scattering. (arXiv:1009.1295v1 [physics.atm-clus])
  

  Bosonic atom-trimer scattering is studied in the unitary limit using momentum-space equations for four-particle transition operators. The impact of the Efimov effect on the atom-trimer scattering observables is explored and a number of universal relations is established. Positions and widths of tetramer resonances are determined.

  
  
  
• Critical Correlations for Short-Range Valence-Bond Wavefunctions on the Square Lattice. (arXiv:1009.1307v1 [cond-mat.str-el])
  

  We investigate the arguably simplest $SU(2)$-invariant wavefunctions capable of accounting for spin-liquid behavior, expressed in terms of nearest-neighbor valence-bond states on the square lattice and characterized by different topological invariants. While such wave-functions are known to exhibit short-range spin correlations, we perform Monte Carlo simulations and show that four-point correlations decay algebraically with an exponent $1.16(4)$. This is reminiscent of the {\it classical} dimer problem, albeit with a slower decay. Furthermore, these correlators are found to be spatially modulated according to a wave-vector related to the topological invariants. We conclude that a recently proposed spin Hamiltonian that stabilizes the here considered wave-function(s) as its (degenerate) ground-state(s) should exhibit gapped spin and gapless non-magnetic excitations.

  
  
  
• Statistical Mechanics of Two-dimensional Foams. (arXiv:1009.1309v1 [cond-mat.soft])
  

  The methods of statistical mechanics are applied to two-dimensional foams under macroscopic agitation. A new variable —the total cell curvature— is introduced, which plays the role of energy in conventional statistical thermodynamics. The probability distribution of the number of sides for a cell of given area is derived. This expression allows to correlate the distribution of sides (“topological disorder”) to the distribution of sizes (“geometrical disorder”) in a foam. The model predictions agree well with available experimental data.

  
  
  
• Bose-Hubbard model with occupation dependent parameters. (arXiv:1009.1313v1 [cond-mat.quant-gas])
  

  We study the ground-state properties of ultracold bosons in an optical lattice in the regime of strong interactions. The system is described by a non-standard Bose-Hubbard model with both occupation-dependent tunneling and on-site interaction parameters. We find that for sufficiently strong coupling, the system features a phase-transition from a Mott insulator with one particle per site to a superfluid of spatially extended particle pairs living on top of the Mott background. Increasing the interaction further, a superfluid of particle pairs localized on a single site (rather than being extended) on top of the Mott background appears. This happens at the same time when the Mott-insulator phase with 2 particles per site is destroyed completely by particle-hole fluctuations for arbitrary small tunneling. In another regime, characterized by weak interaction, but high occupation numbers, we observe a dynamical instability in the superfluid excitation spectrum. The new ground state is a superfluid, forming a 2D slab, localized along one spatial direction that is spontaneously chosen.

  
  
  
• On three-point connectivity in two-dimensional percolation. (arXiv:1009.1314v1 [hep-th])
  

  We argue the exact universal result for the three-point connectivity of critical percolation in two dimensions. Predictions for Potts clusters and for the scaling limit below p_c are also given.

  
  
  
• Scintillation properties of ceramics based on zinc oxide. (arXiv:1009.1321v1 [cond-mat.mtrl-sci])
  

  Ceramics ZnO:Zn of 20mm diameter and 1.6mm thickness with an optical transparency up to 0.33 in the visible region have been prepared by hot pressing technique. Scintillating and luminescent characteristics such as emission spectra, decay time, yield, and TSL glow curve have been measured under X-ray excitation. Two emission bands peaking at 500 and 380 nm were detected, the light output was about 80% of that for standard BGO scintillator, main decay constant was 10.4 0.1 ns. The obtained data allow us to consider the ZnO:Zn ceramics as a perspective scintillator. Finally, the investigation shows that other ZnO-based fast scintillators can be fabricated in the form of optical ceramics.

  
  
  
• Optical, luminescence, and scintillation properties of ZnO and ZnO:Ga ceramics. (arXiv:1009.1324v1 [cond-mat.mtrl-sci])
  

  Uniaxial hot pressing has been used to obtain ceramics based on zinc oxide, and their optical, x-ray-structure, luminescence, and scintillation characteristics have been studied. It is shown that, by changing the concentration of the dopant (Ga) and the codopant (N), it is possible to change the intensities of the edge band (397.5 nm) and the intraband luminescence (510 nm) of the ZnO luminescence, as well as their ratio. Undoped ZnO ceramic has good transparency in the visible region and fairly high luminous yield: 9050 photons per MeV. Ceramic ZnO:Ga possesses intense edge luminescence with a falloff time of about 1 ns.

  
  
  
• Emission and Excitation Spectra of ZnO:Ga and ZnO:Ga,N Ceramics. (arXiv:1009.1325v1 [cond-mat.mtrl-sci])
  

  The spectral characteristics of ZnO:Ga and ZnO:Ga,N ceramics prepared by uniaxial hot pressing have been investigated. At room temperature, the edge (exciton) band at 3.12 eV dominates in the luminescence spectra of ZnO:Ga, while a wide luminescence band at 2.37 eV, which is likely to be due to zinc vacancies, is observed in the spectra of ZnO:Ga,N. Upon heating, the edge band maximum shifts to lower energies and the bandwidth increases. The extrapolated position of the edge-band maximum at zero temperature, Em(0) = 3.367 0.005 eV, is in agreement with the data for thin zinc oxide films. The luminescence excitation spectra in the range from 3 to 6.5 eV are reported and the mechanism of energy transfer to excitons and luminescence centers is considered.

  
  
  
• Luminescence of a ZnO:Ga Crystal upon Excitation in Vacuum UV Region. (arXiv:1009.1327v1 [cond-mat.mtrl-sci])
  

  The spectral–kinetic characteristics of a ZnO:Ga single crystal upon excitation in the vacuum UV region have been studied. At a temperature of 8 K, the exciton luminescence line peaking at 3.356 eV has an extremely small half-width (7.2 meV) and a short decay time (360 ps). In the visible range, a wide luminescence band peaking at ~2.1 eV with a long luminescence time at 8 K and a decay time in the nanosecond range at 300 K is observed. The luminescence excitation spectra of ZnO:Ga have been measured in the range from 4 to 12.5 eV

  
  
  
• Location of the Energy Levels of the Rare-Earth Ion in BaF2 and CdF2. (arXiv:1009.1328v1 [cond-mat.mtrl-sci])
  

  The location of the energy levels of rare-earth (RE) elements in the energy band diagram of BaF2 and CdF2 crystals is determined. The role of RE3+ and RE2+ ions in the capture of charge carriers, luminescence, and the formation of radiation defects is evaluated. It is shown that the substantial difference in the luminescence properties of BaF2:RE and CdF2:RE is associated with the location of the excited energy levels in the band diagram of the crystals.

  
  
  
• Influence of lattice distortion on the Curie temperature and spin-phonon coupling in LaMn$_{0.5}$Co$_{0.5}$O$_{3}$. (arXiv:1009.1330v1 [cond-mat.str-el])
  

  Two distinct ferromagnetic phases of LaMn$_{0.5}$Co$_{0.5}$O$_{3}$ having monoclinic structure with distinct physical properties have been studied. The ferromagnetic ordering temperature $\textit{T}_{c}$ is found to be different for both the phases. The origin of such contrasting characteristics is assigned to the changes in the distance(s) and angle(s) between Mn - O - Co resulting from distortions observed from neutron diffraction studies. Investigations on the temperature dependent Raman spectroscopy provide evidence for such structural characteristics, which affects the exchange interaction. The difference in B-site ordering which is evident from the neutron diffraction is also responsible for the difference in $\textit{T}_{c}$. Raman scattering suggests the presence of spin-phonon coupling for both the phases around the $\textit{T}_{c}$. Electrical transport properties of both the phases have been investigated based on the lattice distortion.

  
  
  
• High On/Off Ratio Graphene Nanoconstriction Field Effect Transistor. (arXiv:1009.1335v1 [cond-mat.mes-hall])
  

  We report a method to pattern monolayer graphene nanoconstriction field effect transistors (NCFETs) with critical dimensions below 10 nm. NCFET fabrication is enabled by the use of feedback controlled electromigration (FCE) to form a constriction in a gold etch mask that is first patterned using conventional lithographic techniques. The use of FCE allows the etch mask to be patterned on size scales below the limit of conventional nanolithography. We observe the opening of a confinement-induced energy gap as the NCFET width is reduced, as evidenced by a sharp increase in the NCFET on/off ratio. The on/off ratios we obtain with this procedure can be larger than 1000 at room temperature for the narrowest devices; this is the first report of such large room temperature on/off ratios for patterned graphene FETs.

  
  
  
• Orientational order and glassy states in networks of semiflexible polymers. (arXiv:1009.1337v1 [cond-mat.soft])
  

  Motivated by the structure of networks of cross-linked cytoskeletal biopolymers, we study the orientationally ordered phases in two-dimensional networks of randomly cross-linked semiflexible polymers. We consider permanent cross-links which prescribe a finite angle and treat them as quenched disorder in a semi-microscopic replica field theory. Starting from a fluid of un-cross-linked polymers and small polymer clusters (sol) and increasing the cross-link density, a continuous gelation transition occurs. In the resulting gel, the semiflexible chains either display long range orientational order or are frozen in random directions depending on the value of the crossing angle, the crosslink concentration and the stiffness of the polymers. A crossing angle $\theta\sim 2\pi/M$ leads to long range $M$-fold orientational order, e.g., ``hexatic'' or ``tetratic'' for $\theta=60^{\circ}$ or $90^{\circ}$, respectively. The transition is discontinuous and the critical cross-link density depends on the bending stiffness of the polymers and the cross-link geometry: the higher the stiffness and the lower $M$, the lower the critical number of cross-links. In between the sol and the long range ordered state, we always observe a gel which is a statistically isotropic amorphous solid (SIAS) with random positional and random orientational localization of the participating polymers.

  
  
  
• Perturbation Theory for Plasmonic Modulation and Sensing. (arXiv:1009.1339v1 [physics.optics])
  

  We develop a general perturbation theory to treat small parameter changes in dispersive plasmonic nanostructures and metamaterials. We specifically apply it to dielectric refractive index, and metallic plasma frequency modulation in metal- dielectric nanostructures. As a numerical demonstration, we verify the theory's accu- racy against direct calculations, for a system of plasmonic rods in air where the metal is defined by a two-pole fit of silver's dielectric function. We also discuss new optical behavior related to plasma frequency modulation in such systems. Our approach provides new physical insight for the design of plasmonic devices for biochemical sensing and optical modulation, and future active metamaterial applications.

  
  
  
• A Hebbian approach to complex network generation. (arXiv:1009.1343v1 [cond-mat.stat-mech])
  

  Through a redefinition of patterns in an Hopfield-like model, we introduce and develop an approach to model discrete systems made up of many, interacting components with inner degrees of freedom. Our approach clarifies the intrinsic connection between the kind of interactions among components and the emergent topology describing the system itself; also, it allows to effectively address the statistical mechanics on the resulting networks. Indeed, a wide class of analytically treatable, weighted random graphs with a tunable level of correlation can be recovered and controlled. We especially focus on the case of imitative couplings among components endowed with similar patterns (i.e. attributes), which, as we show, naturally and without any a-priori assumption, gives rise to small-world effects. We also solve the thermodynamics (at a replica symmetric level) by extending the double stochastic stability technique: free energy, self consistency relations and fluctuation analysis for a picture of criticality are obtained.

  
  
  
• Equilibrium statistical mechanics on correlated random graphs. (arXiv:1009.1345v1 [cond-mat.stat-mech])
  

  Biological and social networks have recently attracted enormous attention between physicists. Among several, two main aspects may be stressed: A non trivial topology of the graph describing the mutual interactions between agents exists and/or, typically, such interactions are essentially (weighted) imitative. Despite such aspects are widely accepted and empirically confirmed, the schemes currently exploited in order to generate the expected topology are based on a-priori assumptions and in most cases still implement constant intensities for links. Here we propose a simple shift in the definition of patterns in an Hopfield model to convert frustration into dilution: By varying the bias of the pattern distribution, the network topology -which is generated by the reciprocal affinities among agents - crosses various well known regimes (fully connected, linearly diverging connectivity, extreme dilution scenario, no network), coupled with small world properties, which, in this context, are emergent and no longer imposed a-priori. The model is investigated at first focusing on these topological properties of the emergent network, then its thermodynamics is analytically solved (at a replica symmetric level) by extending the double stochastic stability technique, and presented together with its fluctuation theory for a picture of criticality. At least at equilibrium, dilution simply decreases the strength of the coupling felt by the spins, but leaves the paramagnetic/ferromagnetic flavors unchanged. The main difference with respect to previous investigations and a naive picture is that within our approach replicas do not appear: instead of (multi)-overlaps as order parameters, we introduce a class of magnetizations on all the possible sub-graphs belonging to the main one investigated: As a consequence, for these objects a closure for a self-consistent relation is achieved.

  
  
  
• Multi-partite entanglement and quantum phase transition in the one-, two-, and three-dimensional transverse field Ising model. (arXiv:1009.1357v1 [quant-ph])
  

  In this paper we consider the quantum phase transition in the Ising model in the presence of a transverse field in one, two and three dimensions from a multi-partite entanglement point of view. Using \emph{exact} numerical solutions, we are able to study such systems up to 25 qubits. The Meyer-Wallach measure of global entanglement is used to study the critical behavior of this model. The transition we consider is between a symmetric GHZ-like state to a paramagnetic product-state. We find that global entanglement serves as a good indicator of quantum phase transition with interesting scaling behavior. We use finite-size scaling to extract the critical point as well as some critical exponents for the one and two dimensional models. Our results indicate that such multi-partite measure of global entanglement shows universal features regardless of dimension $d$. Our results also provides evidence that multi-partite entanglement is better suited for the study of quantum phase transitions than the much studied bi-partite measures.

  
  
  
• Mapping between Hamiltonians with attractive and repulsive potentials on a lattice. (arXiv:1009.1366v1 [cond-mat.quant-gas])
  

  Through a simple and exact analytical derivation, we show that for a particle on a lattice, there is a one-to-one correspondence between the spectra in the presence of an attractive potential $\hat{V}$ and its repulsive counterpart $-\hat{V}$. For a Hermitian potential, this result implies that the number of localized states is the same in both, attractive and repulsive, cases although these states occur above (below) the band-continnum for the repulsive (attractive) case. For a $\mP\mT$-symmetric potential that is odd under parity, our result implies that in the $\mP\mT$-unbroken phase, the energy eigenvalues are symmetric around zero, and that the corresponding eigenfunctions are closely related to each other.

  
  
  
• Relativistic Drude Response of Photoexcited Dirac Quasiparticles in Graphene. (arXiv:1009.1377v1 [cond-mat.mes-hall])
  

  Graphene, a monolayer of carbon atoms arranged in a hexagonal pattern, provides a unique two-dimensional (2D) system exhibiting exotic phenomena such as quantum Hall effects, massless Dirac quasiparticle excitations and universal absorption & conductivity. The linear energy-momentum dispersion relation in graphene also offers the opportunity to mimic the physics of far-away relativistic particles like neutron stars and white dwarfs. In this letter, we perform a counterintuitive ultrafast pump-probe experiment with high photon energies to isolate the Drude-like intraband dynamics of photoexcited carriers. We directly demonstrate the relativistic nature of the photoexcited Dirac quasiparticles by observing a nonlinear scaling of the response with the density of photoexcited carriers. This is in striking contrast to the linear scaling that is usually observed in conventional materials. Our results also indicate strong electron-phonon coupling in graphene, leading to a sub-100 femtosecond thermalization between high energy photoexcited carriers and optical phonons.

  
  
  
• Structure of Optimal Transport Networks Subject to a Global Constraint. (arXiv:cond-mat/0608208v3 [cond-mat.dis-nn] UPDATED)
  

  The structure of pipe networks minimizing the total energy dissipation rate is studied analytically. Among all the possible pipe networks that can be built with a given total pipe volume (or pipe lateral surface area), the network which minimizes the dissipation rate is shown to be loopless. Furthermore, such an optimal network is shown to contain at most N-2 nodes in addition to the N sources plus sinks that it connects. These results are valid whether the possible locations for the additional nodes are chosen freely or from a set of nodes (such as points of a grid). Applications of these results to various physical situations and to the efficient computation of optimal pipe networks are also discussed.

  
  
  
• Structural Properties of Stiff Elastic Networks. (arXiv:0805.4712v2 [cond-mat.soft] UPDATED)
  

  Networks of elastic beams can deform either by stretching or bending of their members. The primary mode of deformation (bending or stretching) crucially depends on the specific details of the network architecture. In order to shed light on the relationship between microscopic geometry and macroscopic mechanics, we characterize the structural features of networks which deform uniformly, through the stretching of the beams only. We provide a convenient set of geometrical criteria to identify such networks, and derive the values of their effective elastic moduli. The analysis of these criteria elucidates the variability of mechanical response of elastic networks. In particular, our study rationalizes the difference in mechanical behavior of cellular and fiber networks.

  
  
  
• Laser Singular Theta-Pinch. (arXiv:0906.0517v8 [nlin.PS] UPDATED)
  

  The interaction of the two counter-propagating ultrashort laser pulses with a singular wavefronts in the thin slice of the underdense plasma is considered. It is shown that ion-acoustic wave is excited via Brillouin three-wave resonance by corkscrew interference pattern of a paraxial singular laser beams. The orbital angular momentum carried by light is transferred to plasma ion-acoustic vortex. The rotation of the density perturbations of electron fluid is the cause of helical current which produce the kilogauss axial quasi-static magnetic field. The exact analytical configurations are presented for an ion-acoustic current field and magnetic induction. The range of experimentally accessible parameters is evaluated.

  
  
  
• AFM Dissipation Topography of Soliton Superstructures in Adsorbed Overlayers. (arXiv:0907.3585v4 [cond-mat.mes-hall] UPDATED)
  

  In the atomic force microscope, the nanoscale force topography of even complex surface superstructures is extracted by the changing vibration frequency of a scanning tip. An alternative dissipation topography with similar or even better contrast has been demonstrated recently by mapping the (x,y)-dependent tip damping but the detailed damping mechanism is still unknown. Here we identify two different tip dissipation mechanisms: local mechanical softness and hysteresis. Motivated by recent data, we describe both of them in a onedimensional model of Moire' superstructures of incommensurate overlayers. Local softness at "soliton" defects yields a dissipation contrast that can be much larger than the corresponding density or corrugation contrast. At realistically low vibration frequencies, however, a much stronger and more effective dissipation is caused by the tip-induced nonlinear jumping of the soliton, naturally developing bistability and hysteresis. Signatures of this mechanism are proposed for experimental identification.