Variational Monte Carlo for Interacting Electrons in Quantum Dots

We use a variational Monte Carlo algorithm to solve the electronic structure of two-dimensional semiconductor quantum dots in external magnetic field. We present accurate many-body wave functions for the system in various magnetic field regimes. We show the importance of symmetry, and demonstrate how it can be used to simplify the variational wave functions. We present in detail the algorithm for efficient wave function optimization. We also present a Monte Carlo -based diagonalization technique to solve the quantum dot problem in the strong magnetic field limit where the system is of a multiconfiguration nature.Comment: 34 pages, proceedings of the 1st International Meeting on Advances in Computational Many-Body Physics, to appear in Journal of Low Temperature Physics (vol. 140, nos. 3/4

Radio continuum imaging of the R CrA star-forming region with the ATCA

The aim of this study is to investigate the nature of radio sources associated with young stellar objects (YSOs) belonging to the R CrA cluster. By combining the centimetre radio data with the wealth of shorter wavelength data accumulated recently we wish to refine estimates of the evolutionary stages of the YSOs. Fluxes and spectral indices for the brightest radio sources were derived from the observations at 3, 6, and 20 cm using the ATCA. Seven of detected sources can be assigned to YSOs, which have counterparts in the X-rays, infrared or submm. One of the YSOs, Radio Source 9, is a Class 0 candidate, and another, IRS 7B, is suggested to be in the Class 0/I transition stage. IRS 7B is associated with extended radio lobes at 6 and 20 cm. The lobes may have a gyrosynchrotron emission component, which could be understood in terms of Fermi accleration in shocks. The Class I objects detected here seem to be a mixed lot. One of these, the wide binary IRS 5, shows a negative spectral index, rapid variability, and a high degree of circular polarisation with $V/I\approx33$ % on one of the days of observation. These signs of magnetic activity suggest that at least one of the binary components has advanced beyond the Class I stage. The radio source without YSO assigment, Radio Source 5, has been suggested to be a brown dwarf. The radio properties, in particular its persistent strong emission, do not support this classification. The radio characteristics of the detected YSOs roughly agree with the scheme where the dominant emission mechanism changes with age. The heterogeneity of the Class I group can possibly be explained by a drastic decline in the jet activity during this stage, which also changes the efficiency of free-free absorption around the protostar.Comment: Accepted for publication in A&A (8 pages, 4 figures, 4 tables

Vortices in quantum droplets: Analogies between boson and fermion systems

The main theme of this review is the many-body physics of vortices in quantum droplets of bosons or fermions, in the limit of small particle numbers. Systems of interest include cold atoms in traps as well as electrons confined in quantum dots. When set to rotate, these in principle very different quantum systems show remarkable analogies. The topics reviewed include the structure of the finite rotating many-body state, universality of vortex formation and localization of vortices in both bosonic and fermionic systems, and the emergence of particle-vortex composites in the quantum Hall regime. An overview of the computational many-body techniques sets focus on the configuration interaction and density-functional methods. Studies of quantum droplets with one or several particle components, where vortices as well as coreless vortices may occur, are reviewed, and theoretical as well as experimental challenges are discussed.Comment: Review article, 53 pages, 53 figure

U-Pb zircon dating of metasedimentary rocks within the Ikkari gold deposit, Central Lapland belt, northern Finland

Abstract. The gold deposit of Ikkari located in Central Lapland belt, near Jeesiö and Rajala villages. Central Lapland belt have been divided to six units from oldest to youngest: Salla group, Kuusamo group, Sodankylä group, Savukoski group, Kittilä suite and Kumpu group. The purpose of this study was to figure out how old metasediments of Ikkari are and what stratigraphy unit they represent by using U-Pb method to zircon grains and comparing these age results to older publications. Mineralogy and lithology of the samples were studied in petrography studies from thin section. Two of the samples are conglomerates and eight are sericite-albite altered quartzites. Major zircon populations of the conglomerates occur ca. 2.015 Ga and maximum deposit ages of them occur 1910 ± 10 Ma (120059_A) and 1919 ± 5 Ma (120071_A). Rocks of Kittilä suite and Savukoski group are probably the local source for the sedimentary material of this age. Major zircon populations of the quartzites occur ca. 1.95–1.90 Ga and maximum deposit ages of them are between 1.90–1.86 Ga. The felsic magmatism in the Lapland Granulite belt is potential source for the material of this age zircon populations. Based on U-Pb ages of the detrital zircon populations of this study, samples can be correlated in to the Kumpu group. The conglomerates are slightly older than the quartzites. Metamorphic overgrowth occurs in some zircon grains.U-Pb ikämääritys Ikkarin kultaesiintymän metasedimenttikivien zirkoneista, Keski-Lapin vyöhykkeellä, Pohjois-Suomessa. Tiivistelmä. Ikkarin kultaesiintymä sijaitsee Keski-Lapin vyöhykkeellä, lähellä Jeesiön ja Rajalan kyliä. KeskiLapin vyöhyke koostuu kuudesta stratigrafisesta yksiköstä, vanhimmasta nuorimpaan Sallan ryhmä, Kuusamon ryhmä, Sodankylän ryhmä, Savukosken ryhmä, Kittilän sviitti ja Kummun ryhmä. Tutkimuksen tarkoituksena oli selvittää, Ikkarin metasedimenttien ikäjakauma ja mihin stratigrafiseen yksikköön ne kuuluvat. Ikämääritykset tehtiin metasedimenteissä olevista detritaalisista zirkoneista zirkon U-Pb menetelmän avulla ja näitä ikämääritystuloksia vertailemalla muihin julkaisuihin. Petrografisissa tutkimuksissa tutkittiin ohuthie istä näytteiden mineraalikoostumus ja kivilaji. Kaksi näytteistä on konglomeraatteja ja kahdeksan serisiitti-albiitti muuttuneita kvartsiitteja. Konglomeraattien suurimmat zirkonipopulaatiot asettuvat noin 2.015 Ga ikään ja niiden maksimi kerrostumisiät ovat 1910 ± 10 Ma (120059_A) ja 1919 ± 5 Ma (120071_A). Luultavasti Savukosken ryhmän ja Kittilän sviitin kivet ovat olleet paikallinen lähde 2.05–2.00 Ga zirkoneille. Kvartsiittien suurimmat zirkonipopulaatiot asettuvat noin 1.95–1.90 Ga ja niiden maksimikerrostumisikä on 1.86–1.90 Ga välillä. Felsinen magmatismi Lapin granuliittivyöhykkeellä on mahdollinen materiaalinlähde näille zirkonipopulaatioille. Verrattaessa työn zirkonipopulaatioita muihin julkaisuihin osoittautuivat kaikki työn näytteet Kummun ryhmän kiviksi, konglomeraattien ollessa kuitenkin hieman vanhempia. Osassa zirkoni rakeista on havaittavissa metamorfista ylikasvu

Modelling line emission of deuterated H_3^+ from prestellar cores

Context: The depletion of heavy elements in cold cores of interstellar molecular clouds can lead to a situation where deuterated forms of H_3^+ are the most useful spectroscopic probes of the physical conditions. Aims: The aim is to predict the observability of the rotational lines of H_2D^+ and D_2H^+ from prestellar cores. Methods: Recently derived rate coefficients for the H_3^+ + H_2 isotopic system were applied to the "complete depletion" reaction scheme to calculate abundance profiles in hydrostatic core models. The ground-state lines of H_2D^+(o) (372 GHz) and D_2H^+(p) (692 GHz) arising from these cores were simulated. The excitation of the rotational levels of these molecules was approximated by using the state-to-state coefficients for collisions with H_2. We also predicted line profiles from cores with a power-law density distribution advocated in some previous studies. Results: The new rate coefficients introduce some changes to the complete depletion model, but do not alter the general tendencies. One of the modifications with respect to the previous results is the increase of the D_3^+ abundance at the cost of other isotopologues. Furthermore, the present model predicts a lower H_2D^+ (o/p) ratio, and a slightly higher D_2H^+ (p/o) ratio in very cold, dense cores, as compared with previous modelling results. These nuclear spin ratios affect the detectability of the submm lines of H_2D^+(o) and D_2H^+(p). The previously detected H_2D^+ and D_2H^+ lines towards the core I16293E, and the H_2D^+ line observed towards Oph D can be reproduced using the present excitation model and the physical models suggested in the original papers.Comment: 10 pages, 11 Figures; ver2: updated some of the Figures, added some references, added an entry to acknowledgement

Various spin-polarization states beyond the maximum-density droplet: a quantum Monte Carlo study

Using variational quantum Monte Carlo method, the effect of Landau-level mixing on the lowest-energy--state diagram of small quantum dots is studied in the magnetic field range where the density of magnetic flux quanta just exceeds the density of electrons. An accurate analytical many-body wave function is constructed for various angular momentum and spin states in the lowest Landau level, and Landau-level mixing is then introduced using a Jastrow factor. The effect of higher Landau levels is shown to be significant; the transition lines are shifted considerably towards higher values of magnetic field and certain lowest-energy states vanish altogether.Comment: 4 pages, 2 figures. Submitted to Phys. Rev.

The kinetic temperature of Barnard 68

We have observed the nearby isolated globule Barnard 68 (B68) in the (J,K)=(1,1) and (2,2) inversion lines of ammonia. The gas kinetic temperature derived from these is T=10+-1.2 K. The observed line-widths are almost thermal: DV=0.181+-0.003 km/s (DVtherm=0.164+-0.010 km/s), supporting the earlier hypothesis that B68 is in hydrostatic equilibrium. The kinetic temperature is an input parameter to the physical cloud model put forward recently, and we discuss the impact of the new value in this context.Comment: 4 pages, 2 figures, accepted for publication in A&A (Letters

Wigner molecules in quantum dots: A quantum Monte Carlo study

We study two-dimensional quantum dots using the variational quantum Monte Carlo technique in the weak-confinement limit where the system approaches the Wigner molecule, i.e., the classical solution of point charges in an external potential. We observe the spin-polarization of electrons followed by a smooth transition to a Wigner-molecule-like state as the confining potential is made weaker.Peer reviewe