The Delay Time Distribution of Tidal Disruption Flares

Recent observations suggest that stellar tidal disruption events (TDE) arestrongly overrepresented in rare, post-starburst galaxies. Several dynamicalmechanisms have been proposed to elevate their TDE rates, ranging from centralstellar overdensities to the presence of supermassive black hole (SMBH)binaries. Another such mechanism, introduced here, is a radial velocityanisotropy in the nuclear star cluster produced during the starburst. These,and other, dynamical hypotheses can be disentangled by comparing observationsto theoretical predictions for the TDE delay time distribution (DTD). We showthat SMBH binaries are a less plausible solution for the post-starburstpreference, as they can only reproduce the observed DTD with extensivefine-tuning. The overdensity hypothesis produces a reasonable match to theobserved DTD (based on the limited data currently available), provided that theinitial stellar density profile created during the starburst, $\rho(r)$, isexceptional in both steepness and normalization. In particular, explaining thepost-starburst preference requires $\rho \propto r^{-\gamma}$ with $\gamma\gtrsim 2.5$, i.e. much steeper than the classic Bahcall-Wolf equilibriumprofile of $\gamma = 7/4$. For "ultrasteep" density cusps ($\gamma \ge 9/4$),we show that the TDE rate decays with time measured since the starburst as$\dot{N} \propto t^{-(4\gamma-9)/(2\gamma-3)} / \ln t$. Radial anisotropiesalso represent a promising explanation, provided that initial anisotropyparameters of $\beta_0 \approx 0.5$ are sustainable against the radial orbitinstability. TDE rates in initially anisotropic cusps will decay roughly as$\dot{N} \propto t^{-\beta_0}$. As the sample of TDEs with well-studied hostgalaxies grows, the DTD will become a powerful tool for constraining theexceptional dynamical properties of post-starburst galactic nuclei.

Deterministic versus stochastic model of reprogramming: new evidence from cellular barcoding technique

Factor-mediated reprogramming of somatic cells towards pluripotency is a low-efficiency process during which only small subsets of cells are successfully reprogrammed. Previous analyses of the determinants of the reprogramming potential are based on average measurements across a large population of cells or on monitoring a relatively small number of single cells with live imaging. Here, we applied lentiviral genetic barcoding, a powerful tool enabling the identification of familiar relationships in thousands of cells. High-throughput sequencing of barcodes from successfully reprogrammed cells revealed a significant number of barcodes from related cells. We developed a computer model, according to which a probability of synchronous reprogramming of sister cells equals 10-30%. We conclude that the reprogramming success is pre-established in some particular cells and, being a heritable trait, can be maintained through cell division. Thus, reprogramming progresses in a deterministic manner, at least at the level of cell lineages.

Spin-order-induced multiferroicity in LiCuFe2(VO4)3 and disorder effects in NaCuFe2(VO4)3

Mixed spin chain compounds, ACuFe2(VO4)3 (A= Li,Na), reach magneticallyordered state at TN ~ 11 K (Li) or ~ 9 K (Na) and experience furthertransformation of magnetic order at T* ~ 7 K (Li) or ~ 5 K (Na), evidenced inmagnetic susceptibility chi and specific heat Cp measurements. While no anomalyhas been detected in dielectric property of NaCuFe2(VO4)3, the step-likefeature precedes a sharp peak in permittivity epsilon at TN in LiCuFe2(VO4)3.These data suggest the spin-order-induced ferroelectricity in Li compound andno such thing in Na compound. On the contrary, the Moessbauer spectroscopystudy suggests similarly wide distribution of hyperfine field in between T* andTN for both the compounds. The first principles calculations also providesimilar values for magnetic exchange interaction parameters in both compounds.These observations lead us to conclude on the crucial role of alkali metalsmobility within the channels of the crystal structure needed to be consideredin explaining the improper multiferroicity in one compound and its absence inother.

High-energy exciton transitions in quasi-two-dimensional cadmium chalcogenide nanoplatelets

Semiconductor nanoparticles of cadmium chalcogenides are known to exhibitpronounced thickness-dependent $E_0$ series of exciton transitions at the$\Gamma$ point of the Brillouin zone (BZ). In this work, we report anexperimental evidence for high-energy series of exciton transitions, whichoriginates from BZ points different from the $\Gamma$ point, in the family ofcadmium chalcogenide quasi-2D nanoplatelets (NPLs). Intensive UV absorptionbands demonstrating a pronounced size effect are observed for CdTe, CdSe, andCdS NPLs in addition to the $E_0$ exciton bands in the visible region. Thesenew bands are attributed to transitions analogous to the $E_1$, $E_1+\Delta_1$,and $E_2$ series observed in bulk crystals. First-principles DFT calculationsof the electronic structure and absorption spectra support this explanation andshow that the main contribution to these optical transitions comes from $X$ and$M$ points of the 2D BZ, which originate from $L$ and $X$ points of the 3D BZ.At the same time, the $E_0$ series of transitions at the $\Gamma$ point is welldescribed by the multiband effective-mass model. The observation of the UVexciton bands reveals tunable optical properties of cadmium chalcogenide NPLsin UV spectral region, which may be interesting for practical applications.

High-pressure behavior of superconducting boron-doped diamond

This work investigates the high-pressure structure of freestandingsuperconducting ($T_{c}$ = 4.3\,K) boron doped diamond (BDD) and how it affectsthe electronic and vibrational properties using Raman spectroscopy and x-raydiffraction in the 0-30\,GPa range. High-pressure Raman scattering experimentsrevealed an abrupt change in the linear pressure coefficients and the grainboundary components undergo an irreversible phase change at 14\,GPa. We showthat the blue shift in the pressure-dependent vibrational modes correlates withthe negative pressure coefficient of $T_{c}$ in BDD. The analysis of x-raydiffraction data determines the equation of state of the BDD film, revealing ahigh bulk modulus of $B_{0}$=510$\pm$28\,GPa. The comparative analysis ofhigh-pressure data clarified that the sp$^{2}$ carbons in the grain boundariestransform into hexagonal diamond.

The superconducting gaps in FeSe studied by soft point-contact Andreev reflection spectroscopy

FeSe single crystals have been studied by soft point-contactAndreev-reflection spectroscopy. Superconducting gap features in thedifferential resistance dV/dI(V) of point contacts such as a characteristicAndreev-reflection double-minimum structure have been measured versustemperature and magnetic field. Analyzing dV/dI within the extended two-gapBlonder-Tinkham-Klapwijk model allows to extract both the temperature andmagnetic field dependence of the superconducting gaps. The temperaturedependence of both gaps is close to the standard BCS behavior. Remarkably, themagnitude of the double-minimum structure gradually vanishes in magnetic field,while the minima position only slightly shifts with field indicating a weakdecrease of the superconducting gaps. Analyzing the dV/dI(V) spectra for 25point contacts results in the averaged gap values = 1.8+/-0.4meV and=1.0+/-0.2 meV and reduced values 2/kTc=4.2+/-0.9 and2/kTc=2.3+/-0.5 for the large (L) and small (S) gap, respectively.Additionally, the small gap contribution was found to be within tens of percentdecreasing with both temperature and magnetic field. No signatures in the dV/dIspectra were observed testifying a gapless superconductivity or presence ofeven smaller gaps.

Electronic structure and magnetic properties of the strong-rung spin-1 ladder compound Rb$_3$Ni$_2$(NO$_3$)$_7$

Small single crystals of Rb$_3$Ni$_2$(NO$_3$)$_7$ were obtained bycrystallization from anhydrous nitric acid solution of rubidium nitrate andnickel nitrate hexahydrate. The basic elements of the crystal structure of thisnew compound are isolated spin-1 two-leg ladders of Ni$^{2+}$-ions connected by(NO$_3$)$^-$ groups. The experimental data show the absence of long rangemagnetic order at T $\geq 2$~K. LDA+U calculations and the detailed analysis ofthe experimental data, i.e. of the magnetic susceptibility, the specific heatin magnetic fields up to 9~T, the magnetization, and of the high-frequencyelectron spin resonance data, enable quantitative estimates of the relevantparameters of the $S=1$ ladders in Rb$_3$Ni$_2$(NO$_3$)$_7$ . The rung-coupling$J_1 = 10.5$~K, the leg-coupling $J_2=1.6$~K, and the uniaxial anisotropy $|A|= 179$~GHz are obtained. The scenario of spin liquid quantum ground state isfurther corroborated by quantum Monte Carlo simulations of the magneticsusceptibility.

Elliptic flow of electrons from heavy-flavor hadron decays in Au+Au collisions at $\sqrt{s_{\rm NN}} = $ 200, 62.4, and 39 GeV

We present measurements of elliptic flow ($v_2$) of electrons from the decaysof heavy-flavor hadrons ($e_{HF}$) by the STAR experiment. For Au+Au collisionsat $\sqrt{s_{\rm NN}} = $ 200 GeV we report $v_2$, for transverse momentum($p_T$) between 0.2 and 7 GeV/c using three methods: the event plane method($v_{2}${EP}), two-particle correlations ($v_2${2}), and four-particlecorrelations ($v_2${4}). For Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 62.4 and39 GeV we report $v_2${2} for $p_T< 2$ GeV/c. $v_2${2} and $v_2${4} arenon-zero at low and intermediate $p_T$ at 200 GeV, and $v_2${2} is consistentwith zero at low $p_T$ at other energies. The $v_2${2} at the two lower beamenergies is systematically lower than at $\sqrt{s_{\rm NN}} = $ 200 GeV for$p_T < 1$ GeV/c. This difference may suggest that charm quarks interact lessstrongly with the surrounding nuclear matter at those two lower energiescompared to $\sqrt{s_{\rm NN}} = 200$ GeV.

Erratum: Global polarization measurement in Au + Au collisions [Phys. Rev. C 76 , 024915 (2007)]

Measurements of jet quenching with semi-inclusive hadron+jet distributions in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV

The STAR Collaboration reports the measurement of semi-inclusivedistributions of charged-particle jets recoiling from a high transversemomentum hadron trigger, in central and peripheral Au+Au collisions at$\sqrt{s_{NN}}$ = 200 GeV. Charged jets are reconstructed with the anti-kTalgorithm for jet radii R between 0.2 and 0.5 and with low infrared cutoff oftrack constituents ($p_T>0.2$ GeV/c). A novel mixed-event technique is used tocorrect the large uncorrelated background present in heavy ion collisions.Corrected recoil jet distributions are reported at mid-rapidity, forcharged-jet transverse momentum $p_T^\mathrm{jet,ch}<30$ GeV/c. Comparison ismade to similar measurements for Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76TeV, to calculations for p+p collisions at $\sqrt{s}$ = 200 GeV based on thePYTHIA Monte Carlo generator and on a Next-to-Leading Order perturbative QCDapproach, and to theoretical calculations incorporating jet quenching. Therecoil jet yield is suppressed in central relative to peripheral collisions,with the magnitude of the suppression corresponding to medium-induced chargedenergy transport out of the jet cone of$2.8\pm0.2\mathrm{(stat)}\pm1.5\mathrm{(sys)}$ GeV/c, for$10<p_T^\mathrm{jet,ch}<20$ GeV/c and R = 0.5. No medium-induced change in jetshape is observed for $R<0.5$. The azimuthal distribution oflow-$p_T^\mathrm{jet,ch}$ recoil jets may be enhanced at large azimuthal anglesto the trigger axis, due to scattering off quasi-particles in the hot QCDmedium. Measurement of this distribution gives a 90% statistical confidenceupper limit to the yield enhancement at large deflection angles in centralAu+Au collisions of $50\pm30\mathrm{(sys)\%}$ of the large-angle yield inp+pcollisions predicted by PYTHIA.