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T. Vogt

Publications and source records attributed to T. Vogt.

At least 19 recordsLinked to original sources
arXiv2024

Helicity oscillations in Rayleigh-B\'enard convection of liquid metal in a cell with aspect ratio 0.5

In this paper, we present numerical and experimental results on helicity oscillations in a liquid-metal Rayleigh-B\'enard (RB) convection cell, with an aspect ratio of 0.5. We find that helicity oscillations occur during transitions of flow states that are characterised by significant changes in the Reynolds number. Moreover, we also observe helicity oscillations at flow conditions where the temporal gradient of the change in the Reynolds number is significantly smaller than that of the helicity. Notably, the helicity oscillations observed during the transient double-roll state exhibit characteristics remarkably similar to those associated with the Tayler Instability.

physics.flu-dyn↗
arXiv2020

Transition from steady to oscillating convection rolls in Rayleigh-Bénard convection under the influence of a horizontal magnetic field

In this study we consider the effect of a horizontal magnetic field on the Rayleigh-Bénard convection in a finite liquid metal layer contained in a cuboid vessel (200 \times 200 \times 40 mm^3). Laboratory experiments are performed for measuring temperature and flow field in the alloy GaInSn at Prandtl number Pr = 0.03 and in a Rayleigh number range 2.3 \times 10^4 < Ra < 2.6 \times 10^5. The field strength is varied up to a maximum value of 320 mT (Ha = 2470, Q = 6.11 \times 10^6). The magnetic field forces the flow to form two-dimensional rolls parallel to the direction of the field lines. The experiments confirm the predictions made by Busse and Clever (J. Mécanique Théorique et Appliquée, 1983) who showed that the application of the horizontal magnetic field extends the range in which steady two-dimensional roll structures exist (Busse balloon) towards higher Ra numbers. A transition from the steady to a time-dependent oscillatory flow occurs when Ra exceeds a critical value for a given Chandrasekhar number Q, which is also equivalent to a reduction of the ratio Q/Ra. Our measurements reveal that the first developing oscillations are clearly of two-dimensional nature, in particular a mutual increase and decrease in the size of adjacent convection rolls is observed without the formation of any detectable gradients in the velocity field along the magnetic field direction. At a ratio of Q/Ra = 1, the first 3D structures appear, which initially manifest themselves in a slight inclination of the rolls with respect to the magnetic field direction. Immediately in the course of this, there arise also disturbances in the spaces between adjacent convection rolls, which are advected along the rolls due to the secondary flow driven by Ekman pumping. The transition to fully-developed three-dimensional structures and then to a turbulent regime takes place with further lowering Q/Ra.

physics.flu-dyn↗
arXiv2018

Laboratory experiments on dynamo action and magnetically triggered flow instabilities

Magnetic fields of planets, stars and galaxies are generated by self-excitation in moving electrically conducting fluids. Once produced, magnetic fields can play an active role in cosmic structure formation by destabilizing rotational flows that would be otherwise hydrodynamically stable. For a long time, both hydromagnetic dynamo action as well as magnetically triggered flow instabilities had been the subject of purely theoretical research. Meanwhile, however, the dynamo effect has been observed in large-scale liquid sodium experiments in Riga, Karlsruhe and Cadarache. In this paper, we summarize the results of liquid metal experiments devoted to the dynamo effect and various magnetic instabilities such as the helical and the azimuthal magnetorotational instability and the Tayler instability. We discuss in detail our plans for a precession-driven dynamo experiment and a large-scale Tayler-Couette experiment using liquid sodium, and on the prospects to observe magnetically triggered instabilities of flows with positive shear.

physics.flu-dyn↗
arXiv2010

Imprinting Light Phase on Matter Wave Gratings in Superradiance Scattering

Superradiance scattering from a Bose-Einstein condensate is studied with a two-frequency pumping beam. We demonstrate the possibility of fully tuning the backward mode population as a function of the locked initial relative phase between the two frequency components of the pumping beam. This result comes from an imprinting of this initial relative phase on two matter wave gratings, formed by the forward mode or backward mode condensate plus the condensate at rest, so that cooperative scattering is affected. A numerical simulation using a semiclassical model agrees with our observations.

quant-ph↗
arXiv2007

Structure and Magnetism of the mono-layer hydrate Na0.3NiO2 0.7H2O

The mono-layer hydrate (Ni-MLH) Na0.3NiO2 0.7H2O was synthesized with an average Ni valence close to one in the bi-layer hydrate (Ni-BLH) Na0.3NiO2 1.3H2O. A weak unsaturated ferromagnetism and divergence of the magnetic susceptibilities of field and zero-field cooled samples of both Ni-MLH and Ni-BLH are observed. However, as a result of the increased 3-dimensional electronic and magnetic character present in Ni-MLH a frequency dependence of the AC magnetic susceptibility indicative of the spin glass-like behavior as seen in Ni-BLH was no longer observed in Ni-MLH.

cond-mat.mtrl-sci↗
arXiv2005

Kondo Insulator description of spin state transition in FeSb2

The thermal expansion and heat capacity of FeSb2 at ambient pressure agrees with a picture of a temperature induced spin state transition within the Fe t_{2g} multiplet. However, high pressure powder diffraction data show no sign of a structural phase transition up to 7GPa. A bulk modulus B=84(3)GPa has been extracted and the temperature dependence of the Gruneisen parameter has been determined. We discuss here the relevance of a Kondo insulator description for this material.

cond-mat.str-el↗
arXiv2005

Structure and Thermodynamics of the Mixed Alkali Alanates

The thermodynamics and structural properties of the hexahydride alanates (M2M'AlH6) with the elpasolite structure have been investigated. A series of mixed alkali alanates (Na2LiAlH6, K2LiAlH6 and K2NaAlH6) were synthesized and found to reversibly absorb and desorb hydrogen without the need for a catalyst. Pressure-composition isotherms were measured to investigate the thermodynamics of the absorption and desorption reactions with hydrogen. Isotherms for catalyzed (4 mol% TiCl3) and uncatalyzed Na2LiAlH6 exhibited an increase in kinetics, but no change in the bulk thermodynamics with the addition of a dopant. A structural analysis using synchrotron x-ray diffraction showed that these compounds favor the Fm-3m space group with the smaller ion (M') occupying an octahedral site. These results demonstrate that appropriate cation substitutions can be used to stabilize or destabilize the material and may provide an avenue to improving the unfavorable thermodynamics of a number of materials with promising gravimetric hydrogen densities.

cond-mat.mtrl-sci↗
arXiv2005

Synthesis and characterization of Na03RhO206H2O - a semiconductor with a weak ferromagnetic component

We have prepared the oxyhydrate Na03RhO206H2O by extracting Na+ cations from NaRhO2 and intercalating water molecules using an aqueous solution of Na2S2O8. Synchrotron X-ray powder diffraction, thermogravimetric analysis (TGA), and energy-dispersive x-ray analysis (EDX) reveal that a non-stoichiometric Na03(H2O)06 network separates layers of edge-sharing RhO6 octahedra containing Rh3+(4d6, S=0) and Rh4+ (4d5, S=1/2). The resistivities of NaRhO2 and Na03RhO206H2O (T < 300) reveal insulating and semi-conducting behavior with activation gaps of 134 meV and 7.8 meV, respectively. Both Na03RhO206H2O and NaRhO2 show paramagnetism at room temperature, however, the sodium-deficient sample exhibits simultaneously a weak but experimentally reproducible ferromagnetic component. Both samples exhibit a temperature-independent Pauli paramagnetism, for NaRhO2 at T > 50 K and for Na03RhO206H2O at T > 25 K. The relative magnitudes of the temperature-independent magnetic susceptibilities, that of the oxide sample being half that of the oxyhydrate, is consistent with a higher density of thermally accessible electron states at the Fermi level in the hydrated sample. At low temperatures the magnetic moments rise sharply, providing evidence of localized and weakl -ordered electronic spins.

cond-mat.mtrl-sci↗
arXiv2005

Synthesis and Structure of the Monolayer Hydrate K03CoO2 x 04H2O

The monolayer hydrate (MLH) K03CoO2 x 04H2O was synthesized from K06CoO2 by extracting K+ cations using K2S2O8 as an oxidant and the subsequent intercalation of water between the layers of edge-sharing CoO6 octahedra. A hexagonal structure (space group P63/mmc) with lattice parameters a = 2.8262(1) Angstrom, c = 13.8269(6) Angstrom similar to the MLH Na036CoO2 x 07H2O was established using high-resolution synchrotron X-ray powder diffraction data. The K/H2O layer in the K-MLH is disordered, which is in contrast to the Na-MLH. At low temperatures metallic and paramagnetic behavior was found.

cond-mat.mtrl-sci↗
arXiv2005

Star clusters dynamics in a laboratory: electrons in an ultracold plasma

Electrons in a spherical ultracold quasineutral plasma at temperature in the Kelvin range can be created by laser excitation of an ultra-cold laser cooled atomic cloud. The dynamical behavior of the electrons is similar to the one described by conventional models of stars clusters dynamics. The single mass component, the spherical symmetry and no stars evolution are here accurate assumptions. The analog of binary stars formations in the cluster case is three-body recombination in Rydberg atoms in the plasma case with the same Heggie's law: soft binaries get softer and hard binaries get harder. We demonstrate that the evolution of such an ultracold plasma is dominated by Fokker-Planck kinetics equations formally identical to the ones controlling the evolution of a stars cluster. The Virial theorem leads to a link between the plasma temperature and the ions and electrons numbers. The Fokker-Planck equation is approximate using gaseous and fluid models. We found that the electrons are in a Kramers-Michie-King's type quasi-equilibrium distribution as stars in clusters. Knowing the electron distribution and using forced fast electron extraction we are able to determine the plasma temperature knowing the trapping potential depth.

quant-ph↗
arXiv2003

Oxygen and Strontium Codoping of La2NiO4: Room Temperature Phase Diagrams

We present a detailed room temperature x-ray powder diffraction study on La2-xSrxNiO4+d with 0<=x<=0.12 and 0<=d<=0.13. For x=0.02, 0.04 and 0.06 the oxygen content phase diagrams of the Sr doped samples show a similar sequence of pure phases and miscibility gaps as for pure La2NiO4+d. We find a weak Sr doping dependence of the d-range for the pure LTO, LTT and HTT phases; but overall, the d-ranges of the different phases do not vary strongly for x<=0.06. Drastic changes are observed for x=0.08 and 0.12, where miscibility gaps successively disappear. For x=0.12 all oxygen doped samples are in the HTT phase. The mechanism responsible for the suppression of the phase separation seems to involves multiple factors, including the Coulomb interaction between Sr impurities and interstitial oxygens as well as the reduction of the NiO6 octahedral tilt angle. The doping dependence of the lattice parameters shows clear differences for pure Sr and pure O doping. With the exception of the LTO phase, the in-plane lattice parameters explicitly depend on the type of dopant, rather than the net hole content, p=x+2d. In contrast, the orthorhombic strain in the LTO phase as well as the c-axis length appears to depend only on p; however, in the case of the c-axis length this ''universal'' behavior turns out to be accidental. Our results also show that the chemical pressure of La-site dopants is highly anisotropic, whereas that of O interstitials appears to be more isotropic. In general, this study reveals that Sr-doped samples have to be annealed carefully to achieve d=0, and to permit the study of the intrinsic properties of La2-xSrxNiO4.

cond-mat.supr-con↗
arXiv2003

The role of the lattice in the $γ\to α$ phase transition of Ce: a high pressure neutron and x-ray diffraction study

The temperature and pressure dependence of the thermal displacements and lattice parameters were obtained across the $γ\to α$ phase transition of Ce using high-pressure, high-resolution neutron and synchrotron x-ray powder diffraction. The estimated vibrational entropy change per atom in the $γ\to α$ phase transition, $ΔS^{γ- α}_{\rm vib} \approx (0.75 \pm 0.15)$k$_{\rm B}$, is about half of the total entropy change. The bulk modulus follows a power-law pressure dependence which is well described using the framework of electron-phonon coupling. These results clearly demonstrate the importance of lattice vibrations, in addition to the spin and charge degrees of freedom, for a complete description of the $γ\to α$ phase transition in elemental Ce.

cond-mat.str-el↗
arXiv2003

Positional disorder of Ba in the thermoelectric germanium clathrate Ba6Ge25

The local structure of Ba6Ge25 has been studied by x-ray diffraction and the atomic pair distribution function technique at 40 K and room temperature. Unambiguous evidence has been found that two out of three types of Ba atoms in Ba6Ge25 move off their positions and become locked in split sites at low temperatures.

cond-mat.mtrl-sci↗
arXiv2003

Temperature dependent total scattering structural study of CaCu3Ti4O12

X-ray and neutron powder diffraction data as a function of temperature are analyzed for the colossal dielectric constant material CaCu3Ti4O12. The local structure is studied using atomic pair distribution function analysis. No evidence is found for enhanced oxygen displacement parameters suggesting that short-range octahedral tilt disorder is minimal. However, an unusual temperature dependence for the atomic displacement parameters of calcium and copper is observed. Temperature dependent modeling of the structure, using bond valence concepts, suggests that the calcium atoms become underbonded below approximately 260 K, which provides a rationale for the unusually high Ca displacement parameters at low temperature.

cond-mat.mtrl-sci↗
arXiv2002

Pressure-Induced Intermediate-to-Low Spin State Transition in LaCoO3

Synchrotron X-ray powder diffraction experiments reveal that the transition from a magnetic intermediate spin (IS) state to a nonmagnetic low-spin (LS) ground state in LaCoO3 normally observed when cooling, manifests itself under pressure by an anomalously low bulk compressibility of 150(2) GPa and an initially very large bond compressibility of 4.8e-3 GPa-1 which levels off near 4 GPa. The continuous depopulation of the IS state is driven by an increased crystal field splitting resulting in an effective reduction of the size of the Co3+ cation.

cond-mat.mtrl-sci↗
arXiv2002

Valence electron distribution in MgB2 studied by accurate diffraction measurements and first principle calculations

We use synchrotron x-ray and precision electron diffraction techniques to determine accurately the structure factors of selected reflections that are sensitive to the valence electron distribution in the superconductor MgB2. These values deviate significantly from calculated structure factors using the scattering factors of free atoms, but agree well with our calculated structure factors based on density functional theory. Having experimentally established the reliability of our firstprinciple calculated structure factors, we present electron density maps of the redistribution of the valence electrons that takes place when hypothetical free atoms of Mg and B in MgB2 interact to form the real crystal.

cond-mat.supr-con↗
arXiv2002

Charge transfer in the high dielectric constant materials CaCu3Ti4O12 and CdCu3Ti4O12

The cubic perovskite-related ceramic CaCu3Ti4O12 has a very high static dielectric constant ~10000 at room temperature (RT), which drops to about 100 below about 100 K. Substituting Cd for Ca reduces the RT value of the dielectric constant by over an order of magnitude. The large the dielectric constant may be due to an internal barrier layer capacitance (IBLC) effect. Infrared optical properties show a low-frequency mode that increases dramatically in strength at low temperature, suggesting a change in the effective charges and a breakdown of the IBLC model due to a semiconductor-to-insulator transition.

cond-mat.str-el↗