Signatures of Wigner molecule formation in interacting Dirac fermion quantum dots

We study $N$ interacting massless Dirac fermions confined in a two-dimensional quantum dot. Physical realizations of this problem include a graphene monolayer and the surface state of a strong topological insulator. We consider both a magnetic confinement and an infinite mass confinement. The ground state energy is computed as a function of the effective interaction parameter $\alpha$ from the Hartree-Fock approximation and, alternatively, by employing the M\"uller exchange functional. For N=2, we compare those approximations to exact diagonalization results. The Hartree-Fock energies are highly accurate for the most relevant interaction range \alpha\alt 2, but the M\"uller functional leads to an unphysical instability when \alpha\agt 0.756. Up to 20 particles were studied using Hartree-Fock calculations. Wigner molecule formation was observed for strong but realistic interactions, accompanied by a rich peak structure in the addition energy spectrum.Comment: 8 pages, 8 figure

Exotic superfluid states of ultra-cold Fermi gases

This is a study of ultra-cold Fermi gases in different systems. This thesis is focused on exotic superfluid states, for an example on the three component Fermi gas and the FFLO phase in optical lattices. In the two-components case, superfluidity is studied mainly in the case of the spin population imbalanced Fermi gases and the phase diagrams are calculated from the mean-field theory. Different methods to detect different phases in optical lattices are suggested. In the three-component case, we studied also the uniform gas and harmonically trapped system. In this case, the BCS theory is generalized to three-component gases. It is also discussed how to achieve the conditions to get an SU(3)-symmetric Hamiltonian in optical lattices. The thesis is divided in chapters as follows: Chapter 1 is an introduction to the field of cold quantum gases. In chapter 2 optical lattices and their experimental characteristics are discussed. Chapter 3 deals with two-components Fermi gases in optical lattices and the paired states in lattices. In chapter 4 three-component Fermi gases with and without a harmonic trap are explored, and the pairing mechanisms are studied. In this chapter, we also discuss three-component Fermi gases in optical lattices. Chapter 5 devoted to the higher order correlations, and what they can tell about the paired states. Chapter 6 concludes the thesis.Tässä väitöksessä tutkitaan ultrakylkien fermikaasujen suprajuoksevuutta. Työssä on keskitytty eksoottisiin suprjuokseviin tiloihin, kuten kolmikomponentiseen fermikaasuun ja FFLO-faasiin optisissa hiloissa. Kaksikomponenttisen fermikaasun tapauksessa, olemme keskittyneet lähinnä spinpopulaatioepätasapainoisiin fermikaasuihin, ja laskemme faasidiagrammit käyttäen keskeiskenttä approksimaatiota. Erillaisia metodeja havaita erillaiset faasit esitetään. Tutkimme myös kolmikomponenttista fermikaasua loukussa ja ilman loukkua. Kolmikomponenttisessa tapauksessa standardia BCS-teoriaa on yleistetty. Työssä on tutkittu myös kuinka saavuttaa SU(3)-symmetria optisissa hiloissa

Finite-size version of the excitonic instability in graphene quantum dots

By a combination of Hartree-Fock simulations, exact diagonalization, and perturbative calculations, we investigate the ground-state properties of disorder-free circular quantum dots formed in a graphene monolayer. Taking the reference chemical potential at the Dirac point, we study N \leq 15 interacting particles, where the fine structure constant {\alpha} parametrizes the Coulomb interaction. We explore three different theoretical concepts: (i) Sucher's positive projection ("no-pair") approach, (ii) a more general Hamiltonian conserving both N and the number of additional electron-hole pairs, and (iii) the full quantum electrodynamics (QED) problem, where only N is conserved. We find that electron-hole pair production is important for {\alpha} 1. This corresponds to a reconstruction of the filled Dirac sea and is a finite-size version of the bulk excitonic instability. We also address the effects of an orbital magnetic field.Comment: 9 pages, 10 figures, to appear in PR

FFLO state in 1-, 2- and 3-dimensional optical lattices combined with a non-uniform background potential

We study the phase diagram of an imbalanced two-component Fermi gas in optical lattices of 1-3 dimensions (1D-3D), considering the possibilities of the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO), Sarma/breached pair, BCS and normal states as well as phase separation, at finite and zero temperatures. In particular, phase diagrams with respect to average chemical potential and the chemical potential difference of the two components are considered, because this gives the essential information about the shell structures of phases that will occur in the presence of an additional (harmonic) confinement. These phase diagrams in 1D, 2D and 3D show in a striking way the effect of Van Hove singularities on the FFLO state. Although we focus on population imbalanced gases, the results are relevant also for the (effective) mass imbalanced case. We demonstrate by LDA calculations that various shell structures such as normal-FFLO-BCS-FFLO-normal, or FFLO-normal, are possible in presence of a background harmonic trap. The phases are reflected in noise correlations: especially in 1D the unpaired atoms leave a clear signature of the FFLO state as a zero-correlation area ('breach') within the Fermi sea. This strong signature occurs both for a 1D lattice as well as for a 1D continuum. We also discuss the effect of Hartree energies and the Gorkov correction on the phase diagrams.We study the phase diagram of an imbalanced two-component Fermi gas in optical lattices of 1-3 dimensions (1D-3D), considering the possibilities of the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO), Sarma/breached pair, BCS and normal states as well as phase separation, at finite and zero temperatures. In particular, phase diagrams with respect to average chemical potential and the chemical potential difference of the two components are considered, because this gives the essential information about the shell structures of phases that will occur in the presence of an additional (harmonic) confinement. These phase diagrams in 1D, 2D and 3D show in a striking way the effect of Van Hove singularities on the FFLO state. Although we focus on population imbalanced gases, the results are relevant also for the (effective) mass imbalanced case. We demonstrate by LDA calculations that various shell structures such as normal-FFLO-BCS-FFLO-normal, or FFLO-normal, are possible in presence of a background harmonic trap. The phases are reflected in noise correlations: especially in 1D the unpaired atoms leave a clear signature of the FFLO state as a zero-correlation area ('breach') within the Fermi sea. This strong signature occurs both for a 1D lattice as well as for a 1D continuum. We also discuss the effect of Hartree energies and the Gorkov correction on the phase diagrams.We study the phase diagram of an imbalanced two-component Fermi gas in optical lattices of 1-3 dimensions (1D-3D), considering the possibilities of the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO), Sarma/breached pair, BCS and normal states as well as phase separation, at finite and zero temperatures. In particular, phase diagrams with respect to average chemical potential and the chemical potential difference of the two components are considered, because this gives the essential information about the shell structures of phases that will occur in the presence of an additional (harmonic) confinement. These phase diagrams in 1D, 2D and 3D show in a striking way the effect of Van Hove singularities on the FFLO state. Although we focus on population imbalanced gases, the results are relevant also for the (effective) mass imbalanced case. We demonstrate by LDA calculations that various shell structures such as normal-FFLO-BCS-FFLO-normal, or FFLO-normal, are possible in presence of a background harmonic trap. The phases are reflected in noise correlations: especially in 1D the unpaired atoms leave a clear signature of the FFLO state as a zero-correlation area ('breach') within the Fermi sea. This strong signature occurs both for a 1D lattice as well as for a 1D continuum. We also discuss the effect of Hartree energies and the Gorkov correction on the phase diagrams.Peer reviewe

Kuopio birth cohort - design of a Finnish joint research effort for identification of environmental and lifestyle risk factors for the wellbeing of the mother and the newborn child

Background: A Finnish joint research effort Kuopio Birth Cohort (KuBiCo) seeks to evaluate the effects of genetics, epigenetics and different risk factors (medication, nutrition, lifestyle factors and environmental aspects) during pregnancy on the somatic and psychological health status of the mother and the child. Methods: KuBiCo will ultimately include information on 10,000 mother-child pairs who have given their informed consent to participate in this cohort. Identification of foetal health risk factors that can potentially later manifest as disease requires a repository of relevant biological samples and a flexible open up-to-date data handling system to register, store and analyse biological, clinical and questionnaire-based data. KuBiCo includes coded questionnaire-based maternal background data gathered before, during and after the pregnancy and bio-banking of maternal and foetal samples that will be stored in deep freezers. Data from the questionnaires and biological samples will be collected into one electronic database. KuBiCo consists of several work packages which are complementary to each other: Maternal, foetal and placental metabolism and omits; Paediatrics; Mental wellbeing; Prenatal period and delivery; Analgesics and anaesthetics during peripartum period; Environmental effects; Nutrition; and Research ethics. Discussion: This report describes the set-up of the KuBiCo and descriptive analysis from 3532 parturients on response frequencies and feedback to KuBiCo questionnaires gathered from June 2012 to April 2016. Additionally, we describe basic demographic data of the participants (n = 1172). Based on the comparison of demographic data between official national statistics and our descriptive analysis, KuBiCo represents a cross-section of Finnish pregnant women.Peer reviewe

Tuotantotyökalun ohjaus- ja mittaliitännän toteuttaminen

Insinöörityön tavoitteena oli kehittää ja testata Sintrol Oy:n kehitteillä olevan Production Tool -tuotantotyökalun signaaleja ohjaavat ja mittaavat kytkennät. Tuotantotyökalun tarkoitus on automatisoinnin avulla nopeuttaa tuotantoa ja minimoida inhimillisiä virheitä. Tuotantotyökalun laajuuden takia tuli ensin valita mitä kytkentöjä aiotaan liittää työhön. Työhön valittiin virtamittauskytkentä sekä mittauksen ohjaus. Virtamittauksen pohjaksi tuli Sintrol Oy:n muissa tuotteissa käytetty jännitteennostin, josta syötetään jännite tunnetun vastuksen läpi ja lasketaan virta linjassa. Kytkimien avulla saa-daan vaihdettua virran suunta eli se, mitataanko tuotteelle menevän signaalin arvo vai tuot-teelta tuleva virta. Mittauksen ohjaus suoritetaan syöttämällä signaaligeneraattorista signaalia vaimentimen läpi. Vaimennettu signaali syötetään testattavan tuotteen mittapäälle ja mitattu arvo luetaan tietokoneelta. Työn lopputuloksena oli piirilevy, jossa on virtamittauskytkentä ja vaimennin, jonka alihank-kija on suunnitellut ja testannut.The goal of the thesis was to create a test circuit for the connections of the Production Tool. Production Tool is an upcoming product of Sintrol Oy. Production Tool tests and produces Sintrol’s dust measuring devices. The subject is quite broad so first it was needed to reduce the size of the work. Current measuring circuit and control for the dust measurement were chosen for this work. The current circuit is based on a voltage booster that is used in Sintrol’s other products. The current flows through a measuring resistor and the voltage difference is measured. When voltage and resistance are known, the current can be calculated. Whether to read from the product or write to the product, can be changed with switches The measurement control is created by running a signal from a signal generator through a attenuator and then driven to the measurement probe of the product. The end result was a circuit board that includes both of the circuits. The attenuator was de-signed and tested by a subcontractor

Appearance of flat surface bands in three-dimensional topological insulators in a ferromagnetic exchange field

Paananen T, Gerber H, Götte M, Dahm T. Appearance of flat surface bands in three-dimensional topological insulators in a ferromagnetic exchange field. New Journal of Physics. 2014;16(3): 33019.We study the properties of the surface states in three-dimensional topological insulators in the presence of a ferromagnetic exchange field. We demonstrate that for layered materials like the surface states on the top surface behave qualitatively different than the surface states at the side surfaces. We show that the group velocity of the surface states can be tuned by the direction and strength of the exchange field. If the exchange field becomes larger than the bulk gap of the material, a phase transition into a topologically nontrivial semimetallic state occurs. In particular, the material becomes a Weyl semimetal, if the exchange field possesses a nonzero component perpendicular to the layers. Associated with the Weyl semimetallic state we show that Fermi arcs appear at the surface. Under certain circumstances either one-dimensional or even two-dimensional surface flat bands can appear. We show that the appearance of these flat bands is related to chiral symmetries of the system and can be understood in terms of topological winding numbers. In contrast to previous systems that have been suggested to possess surface flat bands, the present system has a much larger energy scale, allowing the observation of surface flat bands at room temperature. The flat bands are tunable in the sense that they can be turned on or off by rotation of the ferromagnetic exchange field. Our findings are supported by both numerical results on a finite system as well as approximate analytical results

Magnetically robust topological edge states and flat bands

Paananen T, Dahm T. Magnetically robust topological edge states and flat bands. Phys. Rev. B. 2013;87(19): 195447

Topological flat bands in optical checkerboardlike lattices

Paananen T, Dahm T. Topological flat bands in optical checkerboardlike lattices. Phys. Rev. A. 2015;91(3): 33604.We present comparatively simple two-dimensional and three-dimensional checkerboardlike optical lattices possessing nontrivial topological properties accompanied by topological surface states. By simple tuning of the parameters, these lattices can have a topological insulating phase, a topological semimetallic phase, or a trivial insulating phase. This allows the study of different topological phase transitions within a single cold-atom system. In the topologically nontrivial phases, flat bands appear at the surfaces of the system. These surface states possess short localization lengths such that they are observable even in systems with small lattice dimensions

Tunneling Magnetoresistance Devices Based on Topological Insulators: Ferromagnet–Insulator–Topological-Insulator Junctions Employing Bi2Se3

Götte M, Paananen T, Reiss G, Dahm T. Tunneling Magnetoresistance Devices Based on Topological Insulators: Ferromagnet–Insulator–Topological-Insulator Junctions Employing Bi2Se3. Phys. Rev. Applied. 2014;2(5): 54010.We theoretically investigate tunneling magnetoresistance (TMR) devices, which are probing the spin-momentum coupled nature of surface states of the three-dimensional topological insulator Bi2Se3. Theoretical calculations are performed based on a realistic tight-binding model for Bi2Se3. We study both three-dimensional devices, which exploit the surface states of Bi2Se3, as well as two-dimensional devices, which exploit the edge states of thin Bi2Se3 strips. We demonstrate that the material properties of Bi2Se3 allow a TMR ratio at room temperature of the order of 1000%. Analytical formulas are derived that allow a quick estimate of the achievable TMR ratio in these devices. The devices can be used to measure the spin polarization of the topological surface states as an alternative to spin ARPES. Unlike TMR devices based on magnetic tunnel junctions the present devices avoid the use of a second ferromagnetic electrode whose magnetization needs to be pinned