Influence of Al doping on the critical fields and gap values in magnesium diboride single crystals

The lower ($H_{c1}$) and upper ($H_{c2}$) critical fields of Mg$_{1-x}$Al$_{x}$B$_2$ single crystals (for $x = 0$, 0.1 and $\gtrsim 0.2$) have been deduced from specific heat and local magnetization measurements, respectively. We show that $H_{c1}$ and $H_{c2}$ are both decreasing with increasing doping content. The corresponding anisotropy parameter $\Gamma_{H_{c2}}(0) = H^{ab}_{c2}(0)/H^c_{c2}(0)$ value also decreases from $\sim 5$ in pure MgB$_2$ samples down to $\sim 1.5$ for $x \gtrsim 0.2$ whereas $\Gamma_{H_{c1}}(0)=H^c_{c1}(0)/H^{ab}_{c1}(0)$ remains on the order of 1 in all samples. The small and large gap values have been obtained by fitting the temperature dependence of the zero field electronic contribution to the specific heat to the two gap model for the three Al concentrations. Very similar values have also been obtained by point contact spectroscopy measurements. The evolution of those gaps with Al concentration suggests that both band filling and interband scattering effects are present

On the Phenomenology of Hydrodynamic Shear Turbulence

The question of a purely hydrodynamic origin of turbulence in accretion disks is reexamined, on the basis of a large body of experimental and numerical evidence on various subcritical (i.e., linearly stable) hydrodynamic flows. One of the main points of this paper is that the length scale and velocity fluctuation amplitude which are characteristic of turbulent transport in these flows scale like $Re_m^{-1/2}$, where $Re_m$ is the minimal Reynolds number for the onset of fully developed turbulence. From this scaling, a simple explanation of the dependence of $Re_m$ with relative gap width in subcritical Couette-Taylor flows is developed. It is also argued that flows in the shearing sheet limit should be turbulent, and that the lack of turbulence in all such simulations performed to date is most likely due to a lack of resolution, as a consequence of the effect of the Coriolis force on the large scale fluctuations of turbulent flows. These results imply that accretion flows should be turbulent through hydrodynamic processes. If this is the case, the Shakura-Sunyaev $\alpha$ parameter is constrained to lie in the range $10^{-3}-10^{-1}$ in accretion disks, depending on unknown features of the mechanism which sustains turbulence. Whether the hydrodynamic source of turbulence is more efficient than the MHD one where present is an open question.Comment: 31 pages, 3 figures. Accepted for publication in Ap

Electric field and tip geometry effects on dielectrophoretic growth of carbon nanotube nanofibrils on scanning probes

Single-wall carbon nanotube (SWNT) nanofibrils were assembled onto a variety of conductive scanning probes including atomic force microscope (AFM) tips and scanning tunnelling microscope (STM) needles using positive dielectrophoresis (DEP). The magnitude of the applied electric field was varied in the range of 1-20 V to investigate its effect on the dimensions of the assembled SWNT nanofibrils. Both length and diameter grew asymptotically as voltage increased from 5 to 18 V. Below 4 V, stable attachment of SWNT nanofibrils could not be achieved due to the relatively weak DEP force versus Brownian motion. At voltages of 20 V and higher, low quality nanofibrils resulted from incorporating large amounts of impurities. For intermediate voltages, optimal nanofibrils were achieved, though pivotal to this assembly is the wetting behaviour upon tip immersion in the SWNT suspension drop. This process was monitored in situ to correlate wetting angle and probe geometry (cone angles and tip height), revealing that probes with narrow cone angles and long shanks are optimal. It is proposed that this results from less wetting of the probe apex, and therefore reduces capillary forces and especially force transients during the nanofibril drawing process. Relatively rigid probes (force constant >= 2 N/m) exhibited no perceivable cantilever bending upon wetting and de-wetting, resulting in the most stable process control

Inelastic electron tunneling via molecular vibrations in single-molecule transistors

In single-molecule transistors, we observe inelastic cotunneling features that correspond energetically to vibrational excitations of the molecule, as determined by Raman and infrared spectroscopy. This is a form of inelastic electron tunneling spectroscopy of single molecules, with the transistor geometry allowing in-situ tuning of the electronic states via a gate electrode. The vibrational features shift and change shape as the electronic levels are tuned near resonance, indicating significant modification of the vibrational states. When the molecule contains an unpaired electron, we also observe vibrational satellite features around the Kondo resonance.Comment: 5 pages, 4 figures. Supplementary information available upon reques

Exact analytic results for the Gutzwiller wave function with finite magnetization

We present analytic results for ground-state properties of Hubbard-type models in terms of the Gutzwiller variational wave function with non-zero values of the magnetization m. In dimension D=1 approximation-free evaluations are made possible by appropriate canonical transformations and an analysis of Umklapp processes. We calculate the double occupation and the momentum distribution, as well as its discontinuity at the Fermi surface, for arbitrary values of the interaction parameter g, density n, and magnetization m. These quantities determine the expectation value of the one-dimensional Hubbard Hamiltonian for any symmetric, monotonically increasing dispersion epsilon_k. In particular for nearest-neighbor hopping and densities away from half filling the Gutzwiller wave function is found to predict ferromagnetic behavior for sufficiently large interaction U.Comment: REVTeX 4, 32 pages, 8 figure

Superconductivity at 38 K in the iron arsenide (Ba1-xKx)Fe2As2

The ternary iron arsenide BaFe2As2 becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity up to Tc = 38 K in (Ba1-xKx)Fe2As2 with x = 0.4. The parent compound BaFe2As2 as well as KFe2As2 both crystallize in the tetragonal ThCr2Si2-type structure, which consists of (FeAs)- iron arsenide layers separated by barium or potassium ions. BaFe2As2 is a poor metal and exhibits a SDW anomaly at 140 K. By substituting Ba2+ for K+ ions we have introduced holes in the (FeAs)- layers, which suppress the SDW anomaly and induce superconductivity. This scenario is very similar to the recently discovered arsenide-oxide superconductors. The Tc of 38 K in (Ba1-xKx)Fe2As2 is the highest observed critical temperature in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ThCr2Si2-type structure. Our results suggest, that superconductivity in these systems essentially evolves from the (FeAs)- layers and may occur in other related compounds.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let

GENERALIZED CIRCULAR ENSEMBLE OF SCATTERING MATRICES FOR A CHAOTIC CAVITY WITH NON-IDEAL LEADS

We consider the problem of the statistics of the scattering matrix S of a chaotic cavity (quantum dot), which is coupled to the outside world by non-ideal leads containing N scattering channels. The Hamiltonian H of the quantum dot is assumed to be an M x N hermitian matrix with probability distribution P(H) ~ det[lambda^2 + (H - epsilon)^2]^[-(beta M + 2- beta)/2], where lambda and epsilon are arbitrary coefficients and beta = 1,2,4 depending on the presence or absence of time-reversal and spin-rotation symmetry. We show that this ``Lorentzian ensemble'' agrees with microscopic theory for an ensemble of disordered metal particles in the limit M -> infinity, and that for any M >= N it implies P(S) ~ |det(1 - \bar S^{\dagger} S)|^[-(beta M + 2 - beta)], where \bar S is the ensemble average of S. This ``Poisson kernel'' generalizes Dyson's circular ensemble to the case \bar S \neq 0 and was previously obtained from a maximum entropy approach. The present work gives a microscopic justification for the case that the chaotic motion in the quantum dot is due to impurity scattering.Comment: 15 pages, REVTeX-3.0, 2 figures, submitted to Physical Review B

A simple approach to counterterms in N=8 supergravity

We present a simple systematic method to study candidate counterterms in N=8 supergravity. Complicated details of the counterterm operators are avoided because we work with the on-shell matrix elements they produce. All n-point matrix elements of an independent SUSY invariant operator of the form D^{2k} R^n +... must be local and satisfy SUSY Ward identities. These are strong constraints, and we test directly whether or not matrix elements with these properties can be constructed. If not, then the operator does not have a supersymmetrization, and it is excluded as a potential counterterm. For n>4, we find that R^n, D^2 R^n, D^4 R^n, and D^6 R^n are excluded as counterterms of MHV amplitudes, while only R^n and D^2 R^n are excluded at the NMHV level. As a consequence, for loop order L<7, there are no independent D^{2k}R^n counterterms with n>4. If an operator is not ruled out, our method constructs an explicit superamplitude for its matrix elements. This is done for the 7-loop D^4 R^6 operator at the NMHV level and in other cases. We also initiate the study of counterterms without leading pure-graviton matrix elements, which can occur beyond the MHV level. The landscape of excluded/allowed candidate counterterms is summarized in a colorful chart.Comment: 25 pages, 1 figure, published versio

Families of N=2 Strings

In a given 4d spacetime bakcground, one can often construct not one but a family of distinct N=2 string theories. This is due to the multiple ways N=2 superconformal algebra can be embedded in a given worldsheet theory. We formulate the principle of obtaining different physical theories by gauging different embeddings of the same symmetry algebra in the same ``pre-theory.'' We then apply it to N=2 strings and formulate the recipe for finding the associated parameter spaces of gauging. Flat and curved target spaces of both (4,0) and (2,2) signatures are considered. We broadly divide the gauging choices into two classes, denoted by alpha and beta, and show them to be related by T-duality. The distinction between them is formulated topologically and hinges on some unique properties of 4d manifolds. We determine what their parameter spaces of gauging are under certain simplicity ansatz for generic flat spaces (R^4 and its toroidal compactifications) as well as some curved spaces. We briefly discuss the spectra of D-branes for both alpha and beta families.Comment: 66+1 pages, 2 tables, latex 2e, hyperref. ver2: typos corrected, reference adde

Long-range chromosomal interactions increase and mark repressed gene expression during adipogenesis

Obesity perturbs central functions of human adipose tissue, centred on differentiation of preadipocytes to adipocytes, i.e., adipogenesis. The large environmental component of obesity makes it important to elucidate epigenetic regulatory factors impacting adipogenesis. Promoter Capture Hi-C (pCHi-C) has been used to identify chromosomal interactions between promoters and associated regulatory elements. However, long range interactions (LRIs) greater than 1 Mb are often filtered out of pCHi-C datasets, due to technical challenges and their low prevalence. To elucidate the unknown role of LRIs in adipogenesis, we investigated preadipocyte differentiation to adipocytes using pCHi-C and bulk and single nucleus RNA-seq data. We first show that LRIs are reproducible between biological replicates, and they increase >2-fold in frequency across adipogenesis. We further demonstrate that genomic loci containing LRIs are more epigenetically repressed than regions without LRIs, corresponding to lower gene expression in the LRI regions. Accordingly, as preadipocytes differentiate into adipocytes, LRI regions are more likely to contain repressed preadipocyte marker genes; whereas these same LRI regions are depleted of actively expressed adipocyte marker genes. Finally, we show that LRIs can be used to restrict multiple testing of the long-range cis-eQTL analysis to identify variants that regulate genes via LRIs. We exemplify this by identifying a putative long range cis regulatory mechanism at the LYPLAL1/TGFB2 obesity locus. In summary, we identify LRIs that mark repressed regions of the genome, and these interactions increase across adipogenesis, pinpointing developmental regions that need to be repressed in a cell-type specific way for adipogenesis to proceed.Peer reviewe