Archive ouverte UNIGE | last documents for author 'Thomas Christen'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Thomas Christen'engNucleation of Kinks at Grain Boundarieshttps://archive-ouverte.unige.ch/unige:91288https://archive-ouverte.unige.ch/unige:91288The decay of metastable states in spatially one-dimensional systems by kink nucleation at the sample boundaries is investigated for a specific bistable reaction-diffusion system. An expression for the nucleation rate is derived in the framework of multidimensional Kramers theory, and the dependence of the rate on the boundary conditions is discussed. It turns out that the question whether homogeneous kink nucleation in the bulk or heterogeneous kink nucleation at the sample boundary predominates depends on the specific type of the boundary.Tue, 24 Jan 2017 12:18:05 +0100Late cardiac tamponade after percutaneous closure of a patent foramen ovalehttps://archive-ouverte.unige.ch/unige:39732https://archive-ouverte.unige.ch/unige:39732We report the case of a 35-year-old man who had a transient ischemic cerebral attack and then underwent a percutaneous closure of the patent foramen ovale (PFO) with a Cardiastar device. One year later, the patient developed a cardiac tamponade due to an important hemorrhagic pericardial effusion. Transoesophageal echocardiography showed that one of the struts had impinged on the aortic root in the region adjacent to the transverse pericardial sinus. Therefore, we speculated that the strut had passed through the aortic wall by slow erosion, leading to the pericardial effusion. Cardiac CT and subsequent surgery confirmed the perforation of the left atrial roof and the aortic root by two struts of the device. This is the first reported case of late cardiac tamponade and underscores the importance of long-term follow-up after PFO closure device implantation.Mon, 25 Aug 2014 15:12:40 +0200Basic Elements of Electrical Conductionhttps://archive-ouverte.unige.ch/unige:4489https://archive-ouverte.unige.ch/unige:4489A theory of the dynamical conductance of mesoscopic conductors is presented. It is applied to mesoscopic capacitors, resonant double barriers, ballistic wires, metallic diffusive wires, and to the Corbino disk and the Hall bar in quantizing magnetic fields. Central to this approach is a discussion of the charge and potential distribution in mesoscopic conductors. It is necessary to take into account the implications of the long-range Coulomb interaction in order to obtain a charge and current conserving theory. We emphasize the low-frequency response. This has the advantage that the approach is of considerable generality. The theory can be used to discuss the self-consistency of the dc-conductance formula. The theory can also be applied to discuss the rectifying (nonlinear) behavior of mesoscopic conductors.Tue, 01 Dec 2009 16:27:43 +0100Dynamic Conductance in Quantum Hall Systemshttps://archive-ouverte.unige.ch/unige:4488https://archive-ouverte.unige.ch/unige:4488In the framework of the edge-channel picture and the scattering approach to conduction, we discuss the low frequency admittance of quantized Hall samples up to second order in frequency. The first-order term gives the leading order phase-shift between current and voltage and is associated with the displacement current. It is determined by the emittance which is a capacitance in a capacitive arrangement of edge channels but which is inductive-like if edge channels predominate which transmit charge between different reservoirs. The second-order term is associated with the charge relaxation. We apply our results to a Corbino disc and to two- and four-terminal quantum Hall bars, and we discuss the symmetry properties of the current response. In particular, we calculate the longitudinal resistance and the Hall resistance as a function of frequency.Tue, 01 Dec 2009 16:27:42 +0100Admittance and Nonlinear Transport in Quantum Wires, Point Contacts, and Resonant Tunneling Barriershttps://archive-ouverte.unige.ch/unige:4487https://archive-ouverte.unige.ch/unige:4487We present a discussion of the admittance (ac-conductance) and nonlinear I-V-characteristic for a number of mesoscopic conductors. Our approach is based on a generalization of the scattering approach which now includes the effects of the (long-range) Coulomb interaction. We discuss the admittance of a wire with an impurity and with a nearby gate. We extend a discussion of the low-frequency admittance of a quantum point contact to investigate the effects of the gates used to form the contact. We discuss the nonlinear I-V characteristic of a resonant double barrier structure and discuss the admittance for the double barrier for a large range of frequencies. Our approach emphasizes the overall conservation of charge (gauge invariance) and current conservation and the resulting sum rules for the admittance matrix and nonlinear transport coefficients.Tue, 01 Dec 2009 16:27:41 +0100Low frequency admittance of quantized Hall conductorshttps://archive-ouverte.unige.ch/unige:4281https://archive-ouverte.unige.ch/unige:4281We present a current and charge conserving theory for the low frequency admittance of a two-dimensional electron gas connected to ideal metallic contacts and subject to a quantizing magnetic field. In the framework of the edge-channel picture, we calculate the admittance up to first order with respect to frequency. The transport coefficients in first order with respect to frequency, which are called emittances, determine the charge emitted into a contact of the sample or a gate in response to an oscillating voltage applied to a contact of the sample or a nearby gate. The emittances depend on the potential distribution inside the sample which is established in response to the oscillation of the potential at a contact. We show that the emittances can be related to the elements of an electro-chemical capacitance matrix which describes a (fictitious) geometry in which each edge channel is coupled to its own reservoir. The particular relation of the emittance matrix to this electro-chemical capacitance matrix depends strongly on the topology of the edge channels: We show that edge channels which connect different reservoirs contribute with a negative capacitance to the emittance. For example, while the emittance of a two-terminal Corbino disc is a capacitance, the emittance of a two-terminal quantum Hall bar is a negative capacitance. The geometry of the edge-channel arrangement in a many-terminal setup is reflected by symmetry properties of the emittance matrix. We investigate the effect of voltage probes and calculate the longitudinal and the Hall resistances of an ideal four-terminal Hall bar for low frequencies.Mon, 30 Nov 2009 16:08:47 +0100Partial Densities of States, Scattering Matrices, and Green's Functionshttps://archive-ouverte.unige.ch/unige:4280https://archive-ouverte.unige.ch/unige:4280The response of an arbitrary scattering problem to quasi-static perturbations in the scattering potential is naturally expressed in terms of a set of local partial densities of states and a set of sensitivities each associated with one element of the scattering matrix. We define the local partial densities of states and the sensitivities in terms of functional derivatives of the scattering matrix and discuss their relation to the Green's function. Certain combinations of the local partial densities of states represent the injectivity of a scattering channel into the system and the emissivity into a scattering channel. It is shown that the injectivities and emissivities are simply related to the absolute square of the scattering wave-function. We discuss also the connection of the partial densities of states and the sensitivities to characteristic times. We apply these concepts to a delta-barrier and to the local Larmor clock.Mon, 30 Nov 2009 16:08:45 +0100Gauge Invariant Nonlinear Electric Transport in Mesoscopic Conductorshttps://archive-ouverte.unige.ch/unige:4279https://archive-ouverte.unige.ch/unige:4279We use the scattering approach to investigate the nonlinear current-voltage characteristic of mesoscopic conductors. We discuss the leading nonlinearity by taking into account the self-consistent nonequilibrium potential. We emphasize conservation of the overall charge and current which are connected to the invariance under a global voltage shift (gauge invariance). As examples, we discuss the rectification coefficient of a quantum point contact and the nonlinear current-voltage characteristic of a resonant level in a double barrier structure.Mon, 30 Nov 2009 16:08:44 +0100Magnetic Field Symmetry of Dynamic Capacitanceshttps://archive-ouverte.unige.ch/unige:4278https://archive-ouverte.unige.ch/unige:4278While the static capacitance matrix is always symmetric and thus an even function of the magnetic field, the dynamic capacitance matrix of multi-terminal samples obeys in general only the weaker Onsager-Casimir symmetry relations. Our results are in accordance with recent experimental observations of asymmetric dynamic capacitances in quantized Hall systems. We predict quantization of the four-terminal resistances of an insulating Hall sample.Mon, 30 Nov 2009 16:08:43 +0100Diffusion controlled initial recombinationhttps://archive-ouverte.unige.ch/unige:4220https://archive-ouverte.unige.ch/unige:4220This work addresses nucleation rates in systems with strong initial recombination. Initial (or `geminate') recombination is a process where a dissociated structure (anion, vortex, kink etc.) recombines with its twin brother (cation, anti-vortex, anti-kink) generated in the same nucleation event. Initial recombination is important if there is an asymptotically vanishing interaction force instead of a generic saddle-type activation barrier. At low temperatures, initial recombination strongly dominates homogeneous recombination. In a first part, we discuss the effect in one-, two-, and three-dimensional diffusion controlled systems with spherical symmetry. Since there is no well-defined saddle, we introduce a threshold which is to some extent arbitrary but which is restricted by physically reasonable conditions. We show that the dependence of the nucleation rate on the specific choice of this threshold is strongest for one-dimensional systems and decreases in higher dimensions. We discuss also the influence of a weak driving force and show that the transport current is directly determined by the imbalance of the activation rate in the direction of the field and the rate against this direction. In a second part, we apply the results to the overdamped sine-Gordon system at equilibrium. It turns out that diffusive initial recombination is the essential mechanism which governs the equilibrium kink nucleation rate. We emphasize analogies between the single particle problem with initial recombination and the multi-dimensional kink-antikink nucleation problem.Mon, 30 Nov 2009 16:07:27 +0100Die Admittanz mesoskopischer Leiterhttps://archive-ouverte.unige.ch/unige:4217https://archive-ouverte.unige.ch/unige:4217Elektrische Strukturen im Submikrometerbereich haben bei tiefen Temperaturen Eigenschaften, die sich stark von denen makroskopischer Leiter unterscheiden. Auch elektrotechnische Größen wie Widerstand, Kapazität und Induktivität von elektronischen Nanometer-Bauelementen werden durch Quanteneigenschaften verändert.Mon, 30 Nov 2009 16:07:25 +0100Reply to the comment on "Breathers and kink-antikink nucleation"https://archive-ouverte.unige.ch/unige:4214https://archive-ouverte.unige.ch/unige:4214A Reply to the Comment by P. Hänggi and F. Marchesoni.Mon, 30 Nov 2009 16:07:22 +0100Low frequency admittance of a quantum point contacthttps://archive-ouverte.unige.ch/unige:4211https://archive-ouverte.unige.ch/unige:4211We present a current and charge conserving theory for the low frequency admittance of a quantum point contact. We derive expressions for the electrochemical capacitance and the displacement current. The latter is determined by the emittance which equals the capacitance only in the limit of vanishing transmission. With the opening of channels the capacitance and the emittance decrease in a steplike manner in synchronism with the conductance steps. For vanishing reflection, the capacitance vanishes and the emittance is negative.Mon, 30 Nov 2009 16:07:19 +0100Nucleation of weakly driven kinkshttps://archive-ouverte.unige.ch/unige:4209https://archive-ouverte.unige.ch/unige:4209We study nucleation of kink-antikink pairs under weak nonequilibrium conditions and in the strong friction limit. We introduce an effective critical nucleus of size s0, which is small compared to the inverse kink density but large compared to a kink size. We evaluate independently the nucleation rate and the kink lifetime from a multidimensional Kramers theory and by studying kink-antikink annihilation processes. We find a kink density which is independent of s0 and of the driving force in this regime. The result is in accordance with the equilibrium kink density obtained from statistical mechanics.Mon, 30 Nov 2009 16:07:17 +0100