arXiv.org: Condensed Matter

cond-mat updates on arXiv.org

• Symmetries of Langevin processes generating functionals. (arXiv:1007.5059v1 [cond-mat.stat-mech])
  

  We present a comprehensive study of the symmetries of the generating functionals of generic Langevin processes with multiplicative colored noise. We treat both Martin-Siggia-Rose-Jenssen-deDominicis and supersymmetric formalisms. We summarize the relations between observables that they imply including fluctuation relations, fluctuation-dissipation theorems, and Schwinger-Dyson equations. Newtonian dynamics and their invariances follow in the vanishing friction limit.

  
  
  
• Superfluidity and dimerization in a multilayered system of fermionic polar molecules. (arXiv:1007.5061v1 [cond-mat.quant-gas])
  

  We consider a layered system of fermionic molecules with permanent dipole moments aligned by an external field. The dipole interactions between fermions in adjacent layers are attractive and induce inter-layer pairing. Due to competition for pairing among adjacent layers, the mean-field ground state of the layered system is a dimerized superfluid, with pairing only between every-other layer. We construct an effective Ising-XY lattice model that describes the interplay between dimerization and superfluid phase fluctuations. In addition to the dimerized superfluid ground state, and hightemperature normal state, at intermediate temperature, we find an unusual dimerized "pseudogap" state with only short-range phase coherence. We propose light scattering experiments to detect dimerization.

  
  
  
• Morphology Characterization of Argon-Mediated Epitaxial Graphene on C-face SiC. (arXiv:1007.5064v1 [cond-mat.mtrl-sci])
  

  Epitaxial graphene layers were grown on the C-face of 4H- and 6H-SiC using an argon-mediated growth process. Variations in growth temperature and pressure were found to dramatically affect the morphological properties of the layers. The presence of argon during growth slowed the rate of graphene formation on the C-face and led to the observation of islanding. The similarity in the morphology of the islands and continuous films indicated that island nucleation and coalescence is the growth mechanism for C-face graphene.

  
  
  
• Three Projected Wave-Functions for a High-Temperature Superconductor. (arXiv:1007.5067v1 [cond-mat.supr-con])
  

  Recent experiments strongly suggest that a Fermi surface reconstruction and multiple Fermi pockets are important common features of the underdoped high-temperature cuprate superconductors. A related theoretical work [Phys. Rev. B 79, 134512 (2009)] has demonstrated that a number of hallmark phenomena observed in the underdoped cuprates appear naturally in the scenario of a paired electron pocket co-existing with unpaired hole pockets. We propose Gutzwiller-projected wave-functions to describe this two-fluid state as well as two competing states in its vicinity. Using Gutzwiller renormalized mean-field theory, we calculate self-consistently the electron pairing gap and Neel magnetization and show that the proposed pseudogap state has a lower energy than a projected antiferromagnetic metal. It is argued that the pseudogap state may be selected by energetics at finite temperatures due to spin fluctuations.

  
  
  
• Morphology of Nanoclusters and Nanopillars Formed in Nonequilibrium Surface Growth for Catalysis Applications. (arXiv:1007.5081v1 [cond-mat.soft])
  

  We consider growth of nanoclusters and nanopillars in a model of surface deposition and restructuring yielding morphologies of interest in designing catalysis applications. Kinetic Monte Carlo numerical modeling yields examples of the emergence of FCC-symmetry surface features, allowing evaluation of the fraction of the resulting active sites with desirable properties, such as (111)-like coordination, as well as suggesting the optimal growth regimes.

  
  
  
• Heavy fermions in an optical lattice. (arXiv:1007.5083v1 [cond-mat.quant-gas])
  

  We employ a mean-field theory to study ground-state properties and transport of a two-dimensional gas of ultracold alklaline-earth metal atoms governed by the Kondo Lattice Hamiltonian plus a parabolic confining potential. In a homogenous system this mean-field theory is believed to give a qualitatively correct description of heavy fermion metals and Kondo insulators: it reproduces the Kondo-like scaling of the quasiparticle mass in the former, and the same scaling of the excitation gap in the latter. In order to understand ground-state properties in a trap we extend this mean-field theory via local-density approximation. We find that the Kondo insulator gap manifests as a shell structure in the trapped density profile. In addition, a strong signature of the large Fermi surface expected for heavy fermion systems survives the confinement, and could be probed in time-of-flight experiments. From a full self-consistent diagonalization of the mean-field theory we are able to study dynamics in the trap. We find that the mass enhancement of quasiparticle excitations in the heavy Fermi liquid phase manifests as slowing of the dipole oscillations that result from a sudden displacement of the trap center.

  
  
  
• Measurement of a Sign-Changing Two-Gap Superconducting Phase in Electron-Doped Ba(Fe_{1-x}Co_x)_2As_2 Single Crystals using Scanning Tunneling Spectroscopy. (arXiv:1007.5086v1 [cond-mat.str-el])
  

  Scanning tunneling spectroscopic studies of $Ba(Fe_{1-x}Co_x)_2As_2$ (x = 0.06, 0.12) single crystals reveal direct evidence for predominantly two-gap superconductivity. These gaps decrease with increasing temperature and vanish above the superconducting transition $T_c$. The two-gap nature and strong quasiparticle scattering interferences at the nesting wave-vectors $(\pm \pi , 0)$ and $(0, \pm \pi)$ are consistent with sign-changing s-wave superconductivity. The excess zero-bias conductance and the large gap-to-$T_c$ ratios may be attributed to significant disorder due to cobalt doping.

  
  
  
• Low Loss Superconducting Titanium Nitride Coplanar Waveguide Resonators. (arXiv:1007.5096v1 [cond-mat.supr-con])
  

  Thin films of TiN were sputter-deposited onto Si and sapphire wafers with and without SiN buffer layers. The films were fabricated into RF coplanar waveguide resonators, and internal quality factor measurements were taken at millikelvin temperatures in both the many photon and single photon limits, i.e. high and low power regimes, respectively. At high power, internal quality factors ($Q_i$'s) higher than $10^7$ were measured for TiN with predominantly a (200)-TiN orientation. Films that showed significant (111)-TiN texture invariably had much lower $Q_i$'s, on the order of $10^5$. Our studies show that the (200)-TiN is favored for growth at high temperature on either bare Si or SiN buffer layers. However, growth on bare sapphire or Si(100) at low temperature resulted in primarily a (111)-TiN orientation. Ellipsometry and Auger measurements indicate that the (200)-TiN growth on the bare Si substrates is correlated with the formation of a thin, $\approx 2$ nm, layer of SiN during the pre-deposition procedure. In the single photon regime, $Q_i$ of these films exceeded $8\times10^5$, while thicker SiN buffer layers led to reduced $Q_i$'s at low power.

  
  
  
• Quantum spin nanotubes -- frustration, competing orders and criticalities. (arXiv:1007.5102v1 [cond-mat.stat-mech])
  

  Recent developments of theoretical studies on spin nanotubes are reviewed, especially focusing on the $S=1/2$ three-leg spin tube. In contrast to the three-leg spin ladder, the tube has a spin gap in case of the regular-triangle unit cellwhen the rung interaction is sufficiently large. The effective theory based on the Hubbard Hamiltonian indicates a quantum phase transition to the gapless spin liquid due to the lattice distortion to an isosceles triangle. This is also supported by the numerical diagonalization and the density matrix renormalization group analyses. Furthermore, combining analytical and numerical approaches, we reveal several novel magnetic-field-induced phenomena: N\'eel, dimer, chiral and/or inhomogeneous orders, new mechanism for the magnetization plateau formation,and others. The recently synthesized spin tube materials are also briefly introduced.

  
  
  
• Frictional scattering and frictional waveguides: achieving persistent superlubricity at high velocity on the nanoscale. (arXiv:1007.5106v1 [cond-mat.mes-hall])
  

  Nanomechanical devices can operate at much higher speeds than their macroscopic analogues, due to low inertia. For example, peak speeds >100m/s have been predicted for carbon nanotube devices. This stimulates our interest in the atomic-scale physics of friction at high velocity. Here we study a model nanosystem consisting of a graphene flake moving freely on a graphite substrate at >100m/s. Using molecular dynamics we discover that ultra-low friction, or superlubricity, is punctuated by high-friction transients as the flake rotates through successive crystallographic alignments with the substrate. We term this phenomenon frictional scattering and show that it is mathematically analogous to Bragg scattering. We also show that frictional scattering can be eliminated by using graphitic nanoribbons as frictional waveguides to constrain the flake rotation, thus achieving persistent superlubricity. Finally, we propose an experimental method to study nanoscale high-velocity friction. These results may guide the design of efficient high-frequency nanomechanical devices.

  
  
  
• Discovery of several large families of Topological Insulator classes with backscattering-suppressed spin-polarized single-Dirac-cone on the surface. (arXiv:1007.5111v1 [cond-mat.mes-hall])
  

  Three dimensional (3D) topological insulators are novel states of quantum matter that feature spin-momentum locked helical Dirac fermions on their surfaces and hold promise to open new vistas in spintronics, quantum computing and fundamental physics. Experimental realization of many of the predicted topological phenomena requires finding multi-variant topological band insulators which can be multiply connected to magnetic semiconductors and superconductors. Here we present our theoretical prediction and experimental discovery of several new topological insulator classes in AB2X4(124), A2B2X5(225), MN4X7(147), A2X2X'(221) [A,B=Pb,Ge,Sb,Bi and M,N=Pb,Bi and X,X'=Chalcogen family]. We observe that these materials feature gaps up to about 0.35eV. Multi-variant nature allows for diverse surface dispersion tunability, Fermi surface spin-vortex or textured configurations and spin-dependent electronic interference signaling novel quantum transport processes on the surfaces of these materials. Our discovery also provides several new platforms to search for topological-superconductivity (arXiv:0912.3341v1 (2009)) in these exotic materials.

  
  
  
• Appearance and disappearance of superconductivity with Fe site Co substitution in SmFe1-xCoxAsO (x = 0.0 to 1.0). (arXiv:1007.5121v1 [cond-mat.supr-con])
  

  We report synthesis, structural details and magnetization of SmFe1-xCoxAsO with x ranging from 0.0 to 1.0 at close interval of 0.10. It is found that Co substitutes fully at Fe site in SmFeAsO in an iso-structural lattice with slightly compressed cell. The parent compound exhibited known spin density wave (SDW) character below 150K. Successive doping of Co at Fe site suppressed the SDW transition for x = 0.05 and later induced superconductivity for x = 0.10, 0.15 and 0.20 respectively at 14, 15.5 and 9K. The appearance of bulk superconductivity is established by wide open isothermal magnetization M(H) loops. For higher content of Co i.e. x >= 0.30, superconductivity is not observed. Clearly the Co substitution at Fe site in SmFe1-xCoxAsO diminishes the Fe SDW character (x=0.05), introduces bulk superconductivity for x from 0.10 to 0.20 and finally becomes a non-superconductor. The SmCoAsO also exhibits a secondary AFM like transition at below around 50 K. The reported AFM ordering of Sm spins is seen from heat capacity Cp(T) at 4.5 K and the same remains invariant with Co doping in SmFe1-xCoxAsO. Further the FM ordering of Co-spins (seen in magnetization measurements) is not evident in Cp(T) studies, suggesting weak correlations between ordered Co-spins in these systems.

  
  
  
• Lattice - dynamics of isotope - mixed crystals. (arXiv:1007.5125v1 [cond-mat.mtrl-sci])
  

  The present paper reviewed the peculiarities of the lattice dynamics of isotope - mixed crystals

  
  
  
• Quorum Percolation in Living Neural Networks. (arXiv:1007.5143v1 [cond-mat.dis-nn])
  

  Cooperative effects in neural networks appear because a neuron fires only if a minimal number $m$ of its inputs are excited. The multiple inputs requirement leads to a percolation model termed {\it quorum percolation}. The connectivity undergoes a phase transition as $m$ grows, from a network--spanning cluster at low $m$ to a set of disconnected clusters above a critical $m$. Both numerical simulations and the model reproduce the experimental results well. This allows a robust quantification of biologically relevant quantities such as the average connectivity $\kbar$ and the distribution of connections $p_k$

  
  
  
• Density matrix renormalization group study of optical conductivity in one-dimensional Mott insulator Sr_2CuO_3. (arXiv:1007.5166v1 [cond-mat.str-el])
  

  Applying newly developed dynamical density matrix renormalization group techniques at zero and finite temperatures to a Hubbard-Holstein model at half-filling, we examine the optical conductivity of a typical one-dimensional Mott insulator Sr_2CuO_3. We find a set of parameters in the Hubbard-Holstein model, which can describe optical conductivity for both Mott-gap excitation in the high-energy region and phonon-assisted spin excitation in the low-energy region. We also find that electron-phonon interaction gives additional broadening in the temperature dependence of the Mott-gap excitation.

  
  
  
• Disordered spinor Bose-Hubbard model. (arXiv:1007.5177v1 [cond-mat.other])
  

  We study the zero temperature phase diagram of the disordered spin-1 Bose-Hubbard model in a 2-dimensional square lattice. To this aim, we use a mean field Gutzwiller ansatz and a probabilistic mean field perturbation theory. The spin interaction induces two different regimes corresponding to a ferromagnetic and antiferromagnetic order. In the ferromagnetic case, the introduction of disorder reproduces analogous features of the disordered scalar Bose-Hubbard model, consisting in the formation of a Bose glass phase between Mott insulator lobes. In the antiferromagnetic regime the phase diagram differs more from the scalar case. Disorder in the chemical potential can lead to the disappearance of Mott insulator lobes with odd integer filling factor and, for sufficiently strong spin coupling, to Bose glass of singlets between even filling Mott insulator lobes. Disorder in the spinor coupling parameter results in the appearance of a Bose glass phase only between the n and n+1 lobes for n odd. Disorder in the scalar Hubbard interaction inhibits Mott insulator regions for occupation larger than a critical value.

  
  
  
• On the corrections to Strong-Stretching Theory for end-confined, charged polymers in a uniform electric field. (arXiv:1007.5185v1 [cond-mat.soft])
  

  We investigate the properties of a system of semi-diluted polymers in the presence of charged groups and counter-ions, by means of self-consistent field theory. We study a system of polyelectrolyte chains grafted to a similarly, as well as an oppositely charged surface, solving a set of saddle-point equations that couple the modified diffusion equation for the polymer partition function to the Poisson-Boltzmann equation describing the charge distribution in the system. A numerical study of this set of equations is presented and comparison is made with previous studies. We then consider the case of semi-diluted, grafted polymer chains in the presence of charge-end-groups. We study the problem with self-consistent field as well as strong-stretching theory. We derive the corrections to the Milner-Witten-Cates (MWC) theory for weakly charged chains and show that the monomer-density deviates from the parabolic profile expected in the uncharged case. The corresponding corrections are shown to be dictated by an Abel-Volterra integral equation of the second kind. The validity of our theoretical findings is confirmed comparing the predictions with the results obtained within numerical self-consistent field theory.

  
  
  
• Current response of ac-driven nanoelectromechanical systems in single-electron tunneling regime. (arXiv:1007.5186v1 [cond-mat.mes-hall])
  

  We investigate electric current in a single-electron tunnelling device weakly coupled to an ac-driven underdamped harmonic nanomechanical oscillator. In the linear regime, the current can respond to the external frequency in a resonant as well as in an anti-resonant fashion. The main resonance is accompanied by an additional resonance at a half of the external frequency.

  
  
  
• Intercepts of the momentum correlation functions in \mu-Bose gas model and their asymptotics. (arXiv:1007.5187v1 [quant-ph])
  

  The so-called \mu-deformed oscillator (or \mu-oscillator) introduced by A.Jannussis, though known for almost two decades, was not yet given sufficient attention. However, some of its unusual properties were demonstrated and it has a potential for application in phenomenology of elementary particles or in quantum optics. In this paper, the corresponding \mu-Bose gas model based on the set of \mu-oscillators is explored. In the framework of this model the intercepts \lambda^(2)(K) and \lambda^(3)(K) of two- and three-particle momentum correlation functions are calculated. To do this, different orders of approximation in the (small) deformation parameter \mu are considered. In our analysis we focus especially on the asymptotic behavior of the obtained intercepts \lambda^(2)(K) and \lambda^(3)(K).

  
  
  
• Drain current modulation in a nanoscale field-effect-transistor channel by single dopant implantation. (arXiv:1007.5190v1 [cond-mat.mtrl-sci])
  

  We demonstrate single dopant implantation into the channel of a silicon nanoscale metal-oxide-semiconductor field-effect-transistor. This is achieved by monitoring the drain current modulation during ion irradiation. Deterministic doping is crucial for overcoming dopant number variability in present nanoscale devices and for exploiting single atom degrees of freedom. The two main ion stopping processes that induce drain current modulation are examined. We employ 500~keV He ions, in which electronic stopping is dominant, leading to discrete increases in drain current and 14~keV P dopants for which nuclear stopping is dominant leading to discrete decreases in drain current.

  
  
  
• Dissimilarities between the electronic structure of chemically doped and chemically pressurized iron pnictides from an angle-resolved photoemission spectroscopy study. (arXiv:1007.5205v1 [cond-mat.supr-con])
  

  We have studied the electronic structure of EuFe2As2-xPx using high resolution angle-resolved photoemission spectroscopy. Upon substituting As with the isovalent P, which leads to a chemical pressure and to superconductivity, we observe a non-rigid-band like change of the electronic structure along the center of the Brillouin zone (BZ): an orbital and kz dependent increase or decrease in the size of the hole pockets near the Gamma - Z line. On the other hand, the diameter of the Fermi surface cylinders at the BZ corner forming electron pockets, hardly changes. This is in stark contrast to p and n-type doped iron pnictides where, on the basis of ARPES experiments, a more rigid-band like behavior has been proposed. These findings indicate that there are different ways in which the nesting conditions can be reduced causing the destabilization of the antiferromagnetic order and the appearance of the superconducting dome.

  
  
  
• Intrinsic and dopant enhanced solid phase epitaxy in amorphous germanium. (arXiv:1007.5209v1 [cond-mat.mtrl-sci])
  

  The kinetics of intrinsic and dopant-enhanced solid phase epitaxy (SPE) is stud- ied in amorphous germanium (a-Ge) layers formed by ion implantation on <100> Ge substrates. The SPE rates were measured with a time-resolved reflectivity (TRR) system between 300 and 540 degC and found to have an activation energy of (2.15 +/- 0.04) eV. To interpret the TRR measurements the refractive indices of the a-Ge layers were measured at the two wavelengths used, 1.152 and 1.532 {\mu}m. For the first time, SPE rate measurements on thick a-Ge layers (>3 {\mu}m) have also been performed to distinguish between bulk and near-surface SPE growth rate behavior. Possible effects of explosive crystallization on thick a-Ge layers are considered. When H is present in a-Ge it is found to have a considerably greater retarding affect on the SPE rate than for similar concentrations in a-Si layers. Hydrogen is found to reduce the pre-exponential SPE velocity factor but not the activation energy of SPE. However, the extent of H indiffusion into a-Ge surface layers during SPE is about one order of magnitude less that that observed for a-Si layers. This is thought to be due to the lack of a stable surface oxide on a-Ge. Dopant enhanced kinetics were measured in a-Ge layers containing uniform concentration profiles of implanted As or Al spanning the concentration regime 1-10 x1019 /cm-3. Dopant compensation effects are also observed in a-Ge layers containing equal concentrations of As and Al, where the SPE rate is similar to the intrinsic rate. Various SPE models are considered in light of these data.

  
  
  
• Nodes in the Order Parameter of Superconducting Iron Pnictides Observed by Infrared Spectroscopy. (arXiv:1007.5215v1 [cond-mat.supr-con])
  

  The temperature and frequency dependences of the conductivity are derived from optical reflection and transmission measurements of electron doped BaFe$_2$As$_2$ crystals and films. The data is consistent with gap nodes or possibly a very small gap in the crossover region between these two possibilities. This can arise when one of the several pockets known to exist in these systems has extended s-wave gap symmetry with an anisotropic piece canceling or nearly so the isotropic part in some momentum direction. Alternatively, a node can be lifted by impurity scattering which reduces anisotropy. We find that the smaller gap on the hole pocket at the $\Gamma$ point in the Brillouin zone is isotropic s-wave while the electron pocket at the $M$ point has a larger gap which is anisotropic and falls in the crossover region.

  
  
  
• Photoluminescence from nanocrystalline graphite monofluoride. (arXiv:1007.5222v1 [cond-mat.mtrl-sci])
  

  We synthesize and study the structural and optical properties of nanocrystalline graphene monofluoride and graphite monofluoride, which are carbon-based wide bandgap materials. Using laser excitations 2.41 - 5.08 eV, we identify six emission modes of graphite monofluoride, spanning the visible spectrum from red to violet. The energy and linewidth of the modes point to defect-induced midgap states as the source of the photoemission. We discuss possible candidates. Our findings open the window to electro-optical applications of graphene fluoride.

  
  
  
• Bosonic Dynamical Mean-Field Theory. (arXiv:1007.5223v1 [cond-mat.quant-gas])
  

  We derive the Bosonic Dynamical Mean-Field equation for bosonic atoms in optical lattices with arbitrary lattice geometry. The equations are presented as a systematic expansion in 1/z, z being the number of lattice neighbors. Hence the theory is applicable in sufficiently high dimensional lattices. We apply the method to a two-component mixture, for which a rich phase diagram with spin-order is revealed.

  
  
  
• Metastability in a nano-bridge based hysteretic DC-SQUID embedded in superconducting microwave resonator. (arXiv:1007.5225v1 [cond-mat.supr-con])
  

  We study the metastable response of a highly hysteretic DC-SQUID made of a Niobium loop interrupted by two nano-bridges. We excite the SQUID with an alternating current and with direct magnetic flux, and find different stability zones forming diamond-like structures in the measured voltage across the SQUID. When such a SQUID is embedded in a transmission line resonator similar diamond structures are observed in the reflection pattern of the resonator. We have calculated the DC-SQUID stability diagram in the plane of the exciting control parameters, both analytically and numerically. In addition, we have obtained numerical simulations of the SQUID equations of motion, taking into account temperature variations and non-sinusoidal current-phase relation of the nano-bridges. Good agreement is found between experimental and theoretical results.

  
  
  
• Jaming and Geometry of Two-Dimensional Foams. (arXiv:1007.5230v1 [cond-mat.soft])
  

  We experimentally probe the vicinity of the jamming point J, located at a density $\phi$ corresponding to random close packing ($\phi_{rcp} = 0.842$), in two dimensional, bidisperse packings of foam bubbles. We vary the density of the foam layer and extract geometrical measures by image analysis. We confirm the predicted scaling of the average contact number Z with $\phi$ and compare the distribution of local contact numbers to a simple model. We further establish that the distribution of areas $p(A)$ strongly depends on $\phi$. Finally, we find that the distribution of contact forces $p(f)$ systematically varies with density.

  
  
  
• Magnetic domain walls displacement : automotion vs. spin-transfer torque. (arXiv:1007.5233v1 [cond-mat.mes-hall])
  

  The magnetization dynamics equation predicts that a domain wall that changes structure should undergo a displacement by itself - automotion - due to the relaxation of the linear momentum that is associated with the wall structure. We experimentally demonstrate this effect in soft nanostrips,transforming under spin transfer torque a metastable asymmetric transverse wall into a vortex wall. Displacements more than three times as large as under spin transfer torque only are measured for 1~ns pulses. The results are explained by analytical and numerical micromagnetics. Their relevance to domain wall motion under spin transfer torque is emphasized.

  
  
  
• Dominant folding pathways of a peptide chain, from ab-initio quantum-mechanical simulations. (arXiv:1007.5235v1 [q-bio.BM])
  

  Using the Dominant Reaction Pathways method, we perform an ab-initio quantum-mechanical simulation of a conformational transition of a peptide chain. The method we propose makes it possible to investigate the out-of-equilibrium dynamics of these systems, without resorting to an empirical representation of the molecular force field. It also allows to study rare transitions involving rearrangements in the electronic structure. By comparing the results of the ab-initio simulation with those obtained employing a standard force field, we discuss its capability to describe the non-equilibrium dynamics of conformational transitions.

  
  
  
• Quantum quenches and off-equilibrium dynamical transition in the infinite dimensional Bose Hubbard model. (arXiv:1007.5238v1 [cond-mat.quant-gas])
  

  We study the off-equilibrium dynamics of the infinite dimensional Bose Hubbard Model after a quantum quench. The dynamics can be analyzed exactly by mapping it to an effective Newtonian evolution. For integer filling, we find a dynamical transition separating regimes of small and large quantum quenches starting from the superfluid state. This transition is very similar to the one found for the fermionic Hubbard model by mean field approximations.

  
  
  
• Binary non-additive hard sphere mixtures: Fluid demixing, asymptotic decay of correlations and free fluid interfaces. (arXiv:1007.5245v1 [cond-mat.soft])
  

  Using a fundamental measure density functional theory we investigate both bulk and inhomogeneous systems of the binary non-additive hard sphere model. For sufficiently large (positive) non-additivity the mixture phase separates into two fluid phases with different compositions. We calculate bulk fluid-fluid coexistence curves for a range of size ratios and non-additivity parameters and find that they compare well to simulation results from the literature. Using the Ornstein-Zernike equation, we investigate the asymptotic, r->infinity, decay of the partial pair correlation functions, g_ij(r). At low densities there occurs a structural crossover in the asymptotic decay between two different damped oscillatory modes with different wavelengths corresponding to the two intra-species hard core diameters. On approaching the fluid-fluid critical point there is Fisher-Widom crossover from exponentially damped oscillatory to monotonic asymptotic decay. Using the density functional we calculate the density profiles for the planar free fluid-fluid interface between coexisting fluid phases. We show that the type of asymptotic decay of g_ij(r) not only determines the asymptotic decay of the interface profiles, but is also relevant for intermediate and even short-ranged behaviour. We also determine the surface tension of the free fluid interface, finding that it increases with non-additivity, and that on approaching the critical point mean-field scaling holds.

  
  
  
• Transport Through Nanostructures with Asymmetric Coupling to the Leads. (arXiv:1007.5247v1 [cond-mat.mes-hall])
  

  Using an approach to open quantum systems based on the effective non-Hermitian Hamiltonian, we fully describe transport properties for a paradigmatic model of a coherent quantum transmitter: a finite sequence of square potential barriers. We consider the general case of asymmetric external barriers and variable coupling strength to the environment. We demonstrate that transport properties are very sensitive to the degree of opening of the system and determine the parameters for maximum transmission at any given degree of asymmetry. Analyzing the complex eigenvalues of the non-Hermitian Hamiltonian, we show a double transition to a super-radiant regime where the transport properties and the structure of resonances undergo a strong change. We extend our analysis to the presence of disorder and to higher dimensions.

  
  
  
• Exotic paired phases in ladders with spin-dependent hopping. (arXiv:1007.5251v1 [cond-mat.str-el])
  

  Fermions in two-dimensions (2D) when subject to anisotropic spin-dependent hopping can potentially give rise to unusual paired states in {\it unpolarized} mixtures that can behave as non-Fermi liquids. One possibility is a fully paired state with a gap for fermion excitations in which the Cooper pairs remain uncondensed. Such a ``Cooper-pair Bose-metal" phase would be expected to have a singular Bose-surface in momentum space. As demonstrated in the context of 2D bosons hopping with a frustrating ring-exchange interaction, an analogous Bose-metal phase has a set of quasi-1D descendent states when put on a ladder geometry. Here we present a density matrix renormalization group (DMRG) study of the attractive Hubbard model with spin-dependent hopping on a two-leg ladder geometry. In our setup, one spin species moves preferentially along the leg direction, while the other does so along the rung direction. We find compelling evidence for the existence of a novel Cooper-pair Bose-metal phase in a region of the phase diagram at intermediate coupling. We further explore the phase diagram of this model as a function of hopping anisotropy, density, and interaction strength, finding a conventional superfluid phase, as well as a phase of paired Cooper pairs with d-wave symmetry, similar to the one found in models of hard-core bosons with ring-exchange. We argue that simulating this model with cold Fermi gases on spin dependent optical lattices is a promising direction for realizing exotic quantum states.

  
  
  
• BaFe_{1.8}Co_{0.2}As_2 thin film hybrid Josephson junctions. (arXiv:1007.5252v1 [cond-mat.supr-con])
  

  Josephson junctions with iron pnictides open the way for fundamental experiments on superconductivity in these materials and their application in superconducting devices. Here, we present hybrid Josephson junctions with a BaFe_{1.8}Co_{0.2}As_2 thin film electrode, an Au barrier and a PbIn counter electrode. The junctions show RSJ-like current-voltage characteristics up to the critical temperature of the counter electrode of about 7.2K. The temperature dependence of the critical current, IC, does not show an Ambegaokar-Baratoff behavior. Well-pronounced Shapiro steps are observed at microwave frequencies of 10-18GHz. Assuming an excess current, I_ex, of 200 {\mu}A at 4.2K we get an effective I_C R_N product of 6 {\mu}V.

  
  
  
• Puzzling aspects of the low-energy kink in the nodal dispersion of copper-oxide superconductors: Insights from Dynamical Mean Field Theory. (arXiv:1007.5268v1 [cond-mat.str-el])
  

  Motivated by the observation in copper-oxide high-temperature superconductors, we investigate the appearance of kinks in the electronic dispersion due to coupling to phonons for a system with strong electronic repulsion. We study a Hubbard model supplemented by an electron-phonon coupling of Holstein type within Dynamical Mean Field Theory (DMFT) utilizing Numerical Renormalization Group as impurity solver. Paramagnetic DMFT solutions in the presence of large repulsion show a kink only for large values of the electron-phonon coupling $\lambda$ or large doping and, contrary to the conventional electron-phonon theory, the position of such a kink can be shifted to energies larger than the renormalized phonon frequency $\omega_0^r$. When including antiferromagnetic correlations we find a stronger effect of the electron-phonon interaction on the electronic dispersion due to a cooperative effect and a visible kink at $\omega_0^r$, even for smaller $\lambda$. Our results provide a scenario of a kink position increasing with doping, which could be related to recent photoemission experiments on Bi-based cuprates.

  
  
  
• Conductance of Quantum Impurity Models from Quantum Monte Carlo. (arXiv:1007.5280v1 [cond-mat.mes-hall])
  

  The conductance of two Anderson impurity models, one with two-fold and another with four-fold degeneracy, representing two types of quantum dots, is calculated using a world-line quantum Monte Carlo (QMC) method. Extrapolation of the imaginary time QMC data to zero frequency yields the linear conductance, which is then compared to numerical renormalization group results in order to assess its accuracy. We find that the method gives excellent results at low temperature (T<Tk) throughout the mixed valence and Kondo regimes, but it is unreliable for higher temperature.

  
  
  
• A hierarchy of topological tensor network states. (arXiv:1007.5283v1 [cond-mat.str-el])
  

  We present a hierarchy of quantum many-body states among which many examples of topological order can be identified by construction. We define these states in terms of a general, basis-independent framework of tensor networks based on the algebraic setting of finite-dimensional Hopf C*-algebras. At the top of the hierarchy we identify ground states of new topological lattice models extending Kitaev's quantum double models [26]. For these states we exhibit the mechanism responsible for their non-zero topological entanglement entropy by constructing a renormalization group flow. Furthermore it is shown that those states of the hierarchy associated with Kitaev's original quantum double models are related to each other by the condensation of topological charges. We conjecture that charge condensation is the physical mechanism underlying the hierarchy in general.

  
  
  
• Giant Faraday rotation in single- and multilayer graphene. (arXiv:1007.5286v1 [cond-mat.mes-hall])
  

  Optical Faraday rotation is one of the most direct and practically important manifestations of magnetically broken time-reversal symmetry. The rotation angle is proportional to the distance traveled by the light, and up to now sizeable effects were observed only in macroscopically thick samples and in two-dimensional electron gases with effective thicknesses of several nanometers. Here we demonstrate that a single atomic layer of carbon - graphene - turns the polarization by several degrees in modest magnetic fields. The rotation is found to be strongly enhanced by resonances originating from the cyclotron effect in the classical regime and the inter-Landau-level transitions in the quantum regime. Combined with the possibility of ambipolar doping, this opens pathways to use graphene in fast tunable ultrathin infrared magneto-optical devices.

  
  
  
• Asymptotically exact probability distribution for the Sinai model with finite drift. (arXiv:1007.5287v1 [cond-mat.stat-mech])
  

  We obtain the exact asymptotic result for the disorder-averaged probability distribution function for a random walk in a biased Sinai model and show that it is characterized by a creeping behavior of the displacement moments with time, <x^n> ~ t^{\mu n} where \mu is dimensionless mean drift. We employ a method originated in quantum diffusion which is based on the exact mapping of the problem to an imaginary-time Schr\"{odinger} equation. For nonzero drift such an equation has an isolated lowest eigenvalue separated by a gap from quasi-continuous excited states, and the eigenstate corresponding to the former governs the long-time asymptotic behavior.

  
  
  
• Quasienergy description of the driven Jaynes-Cummings model. (arXiv:1007.5289v1 [cond-mat.supr-con])
  

  We analyze the driven resonantly coupled Jaynes-Cummings model in terms of a quasienergy approach by switching to a frame rotating with the external modulation frequency and by using the dressed atom picture. A quasienergy surface in phase space emerges whose level spacing is governed by a rescaled effective Planck constant. Moreover, the well-known multiphoton transitions can be reinterpreted as resonant tunneling transitions from the local maximum of the quasienergy surface. Most importantly, the driving defines a quasienergy well which is nonperturbative in nature. The quantum mechanical quasienergy state localized at its bottom is squeezed. In the Purcell limited regime, the potential well is metastable and the effective local temperature close to its minimum is uniquely determined by the squeezing factor. The activation occurs in this case via dressed spin flip transitions rather than via quantum activation as in other driven nonlinear quantum systems such as the quantum Duffing oscillator. The local maximum is in general stable. However, in presence of resonant coherent or dissipative tunneling transitions the system can escape from it and a stationary state arises as a statistical mixture of quasienergy states being localized in the two basins of attraction. This gives rise to a resonant or an antiresonant nonlinear response of the cavity at multiphoton transitions. The model finds direct application in recent experiments with a driven superconducting circuit QED setup.

  
  
  
• Thermalization of Interacting Fermions and Delocalization in Fock space. (arXiv:1007.5306v1 [cond-mat.stat-mech])
  

  By means of exact diagonalization, we investigate the onset of 'eigenstate thermalization' and the crossover to ergodicity in a system of 1D fermions with increasing interaction. We show that the fluctuations in the expectation values of the momentum distribution from eigenstate to eigenstate decrease with increasing coupling strength and system size. It turns out that these fluctuations are proportional to the inverse participation ratio of eigenstates represented in the Fock basis. We demonstrate that eigenstate thermalization should set in even for vanishingly small perturbations in the thermodynamic limit.

  
  
  
• Anomalously large capacitance of a plane capacitor with a two-dimensional electron gas. (arXiv:1007.5308v1 [cond-mat.mes-hall])
  

  In electronic devices where a two-dimensional electron gas (2DEG) comprises one or both sides of a plane capacitor, the resulting capacitance $C$ can be larger than the ``geometric capacitance" $C_g$ determined by the physical separation $d$ between electrodes. This larger capacitance is known to result from the Coulomb correlations between individual electrons within the 2DEG, which lead to a negative thermodynamic density of states (negative compressibility). Experiments on such systems generally operate in the regime where the average spacing between electrons $n^{-1/2}$ in the 2DEG is larger than $d$, and these experiments observe $C > C_g$ by only a few percent. A recent experiment [1], however, has observed $C$ larger than $C_g$ by almost 40% while operating in the regime $nd^2 << 1$. In this paper we argue that at $nd^2 << 1$ correlations between the electronic charge of opposite electrodes become important, and that these correlations dramatically alter the predictions of existing theories. We develop a theory of the capacitance of a capacitor where a 2DEG comprises either one or both electrodes that is valid over the full range of $nd^2$. We show that, in the absence of disorder, the capacitance diverges when $nd^2$ goes to zero. Our results compare favorably with the experiment of Ref. [1] without the use of adjustable parameters.

  
  
  
• The magnetoresistive behavior of Cr-doped manganites Pr0.44Sr0.56MnO3. (arXiv:cond-mat/0212517v2 [cond-mat.mtrl-sci] UPDATED)
  

  A complex structural, magnetic and electric transport investigation shows that the Cr doping on Mn sites in the A-type antiferromagnet Pr0.44Sr0.56MnO3 provokes a non-uniform magnetic state with coexisting FM and AFM regions. Irrespective of the ratio of magnetic phases, the samples exhibit a non-metallic behavior of resistivity and thermopower, pointing to the nanoscopic nature of the phase separation. A particularly large magnetoresistance encountered in a broad range of temperatures for samples with Cr doping of 4 - 6 % supports such idea.

  
  
  
• Structural anomalies associated with the electronic and spin transitions in LnCoO3. (arXiv:cond-mat/0503104v3 [cond-mat.mtrl-sci] UPDATED)
  

  A powder X-ray diffraction study, combined with the magnetic susceptibility and electric transport measurements, was performed on a series of LnCoO3 perovskites (Ln = Y, Dy, Gd, Sm, Nd, Pr and La) over a temperature range 100 - 1000 K. A non-standard temperature dependence of the observed thermal expansion was modelled as a sum of three contributions: (1) Weighted sum of lattice expansions of the cobaltite in the diamagnetic low spin state and in the intermediate (IS) or high (HS) spin state. (2) An anomalous expansion due to the increasing population of excited (IS or HS) states of Co3+ ions at the course of the diamagnetic-paramagnetic transition. (3) An anomalous expansion due to excitations of Co3+ ions to another paramagnetic state accompanied by an insulator-metal transition. The anomalous expansion is governed by parameters that are found to vary linearly with the Ln ionic radius. In the case of the first magnetic transition it is the energy splitting E between the ground low spin state and the excited state, presumably the intermediate spin state. The energy splitting E, determined by a fit of magnetic susceptibility, decreases with temperature. The values of E determined for LaCoO3 and YCoO3 at T = 0 K as 164 K and 2875 K, respectively, fall to zero at T = 230 K for LaCoO3 and 860 K for YCoO3. The second anomalous expansion connected with a simultaneous magnetic and insulator-metal transition is characterized by its center at T = 535 K for LaCoO3 and 800 K for YCoO3. The change of the unit cell volume during each transition is independent on the Ln cation and is about 1% in both cases.

  
  
  
• Valence and spin states in perovskites LaCo0.95M0.05O3 (M = Mg, Ga, Ti). (arXiv:0710.2762v2 [cond-mat.mtrl-sci] UPDATED)
  

  The samples LaCoO3 with dilute substitutions on cobalt sites have been studied using the resistivity, thermopower and magnetic susceptibility measurements over the temperature range up to ~900 K. The Co-site substitution does not affect the magnetic transition at ~100 K and the onset of massive population of hole carriers at ~500 K, characteristic for undoped LaCoO3. On the other hand, the low-temperature transport and magnetism is markedly distinct for samples with extra charge on cobalt ions introduced by the heterovalent dopants (Mg2+, Ti4+) compared to samples with minor non-stoichiometry (LaCoO3, Ga3+ doped sample). Magnetic properties suggest that these extra charges create thermally stable magnetic polarons of total S ~ 2-3. Common features of Co-site doped and La-site doped samples (La1-xSrxCoO3) are discussed

  
  
  
• A pedestrian's view on interacting particle systems, KPZ universality, and random matrices. (arXiv:0803.2796v2 [cond-mat.stat-mech] UPDATED)
  

  These notes are based on lectures delivered by the authors at a Langeoog seminar of SFB/TR12 "Symmetries and universality in mesoscopic systems" to a mixed audience of mathematicians and theoretical physicists. After a brief outline of the basic physical concepts of equilibrium and nonequilibrium states, the one-dimensional simple exclusion process is introduced as a paradigmatic nonequilibrium interacting particle system. The stationary measure on the ring is derived and the idea of the hydrodynamic limit is sketched. We then introduce the phenomenological Kardar-Parisi-Zhang (KPZ) equation and explain the associated universality conjecture for surface fluctuations in growth models. This is followed by a detailed exposition of a seminal paper of Johansson that relates the current fluctuations of the totally asymmetric simple exclusion process (TASEP) to the Tracy-Widom distribution of random matrix theory. The implications of this result are discussed within the framework of the KPZ conjecture.

  
  
  
• Properties of the hole and electron doped perovskites LnCoO3. (arXiv:0804.2685v3 [cond-mat.mtrl-sci] UPDATED)
  

  Two extreme members of the cobaltite series, LaCoO3 and DyCoO3, were investigated by the electrical resistivity and thermopower measurements up to 800-1000 K. Special attention was given to effects of extra holes or electrons, introduced by light doping of Co sites by Mg2+ or Ti4+ ions. The experiments on the La based compounds were complemented with magnetic measurements. The study shows that both kinds of charge carriers induce magnetic states on surrounding CoIII sites and form thus thermally stable polarons of large total spin. Their itinerancy is characterized by low temperature resistivity, which is of Arrhenius type r~exp(EA/kT) for the hole (CoIV) doped samples, while an unusual dependence r~1/Tn (n=8-10) is observed for the electron (CoII) doped samples. At higher temperatures, additional hole carriers are massively populated in the CoIII background, leading to a resistivity drop. This transition become evident at ~300 K and 450 K and culminates at TI-M=540 and 780 K for the La and Dy based samples, respectively.

  The electronic behaviours of the cobaltites are explained considering two excitation processes in parent compounds. The first one is related to a local excitation from the diamagnetic LS CoIII to close-lying paramagnetic HS CoIII state. Secondarily, a metallic phase of the IS CoIII character is formed through a charge transfer mechanism between LS/HS pairs. The magnetic polarons associated with doped carriers are interpreted as droplets of such IS phase.

  
  
  
• Graphs, links, and duality on surfaces. (arXiv:0903.5312v3 [math.CO] UPDATED)
  

  We introduce a polynomial invariant of graphs on surfaces, $P_G$, generalizing the classical Tutte polynomial. Topological duality on surfaces gives rise to a natural duality result for $P_G$, analogous to the duality for the Tutte polynomial of planar graphs. This property is important from the perspective of statistical mechanics, where the Tutte polynomial is known as the partition function of the Potts model. For ribbon graphs, $P_G$ specializes to the well-known Bollobas-Riordan polynomial, and in fact the two polynomials carry equivalent information in this context. Duality is also established for a multivariate version of the polynomial $P_G$. We then consider a 2-variable version of the Jones polynomial for links in thickened surfaces, taking into account homological information on the surface. An analogue of Thistlethwaite's theorem is established for these generalized Jones and Tutte polynomials for virtual links.

  
  
  
• Self-gravitating Brownian particles in two dimensions: the case of N=2 particles. (arXiv:0911.1022v2 [cond-mat.stat-mech] UPDATED)
  

  We study the motion of N=2 overdamped Brownian particles in gravitational interaction in a space of dimension d=2. This is equivalent to the simplified motion of two biological entities interacting via chemotaxis when time delay and degradation of the chemical are ignored. This problem also bears some similarities with the stochastic motion of two point vortices in viscous hydrodynamics [Agullo & Verga, Phys. Rev. E, 63, 056304 (2001)]. We analytically obtain the density probability of finding the particles at a distance r from each other at time t. We also determine the probability that the particles have coalesced and formed a Dirac peak at time t (i.e. the probability that the reduced particle has reached r=0 at time t). Finally, we investigate the variance of the distribution <r^2> and discuss the proper form of the virial theorem for this system. The reduced particle has a normal diffusion behaviour for small times with a gravity-modified diffusion coefficient <r^2>=r_0^2+(4k_B/\xi\mu)(T-T_*)t, where k_BT_{*}=Gm_1m_2/2 is a critical temperature, and an anomalous diffusion for large times <r^2>~t^(1-T_*/T). As a by-product, our solution also describes the growth of the Dirac peak (condensate) that forms in the post-collapse regime of the Smoluchowski-Poisson system (or Keller-Segel model) for T<T_c=GMm/(4k_B). We find that the saturation of the mass of the condensate to the total mass is algebraic in an infinite domain and exponential in a bounded domain.

  
  
  
• Effects of polarization on the band-structure of delafossite transparent conductive oxides. (arXiv:0912.0618v2 [cond-mat.mtrl-sci] UPDATED)
  

  We use hybrid functionals and restricted self-consistent GW, state-of-the-art theoretical approaches for quasiparticle band structures, to study the electronic states of delafossite Cu(Al,In)O$_2$, the first p-type and bipolar transparent conductive oxides. We show that self-consistent GW gives remarkably wider band gaps than all the other approaches used so far. Accounting for polaronic effects in the GW scheme we recover a very nice agreement with experiments. Furthermore, the modifications with respect to the Kohn-Sham bands are strongly k-dependent, which makes questionable the common practice of using a scissor operator. Finally, our results support the view that the low energy structures found in optical experiments, and initially attributed to an indirect transition, are due to intrinsic defects in the samples.