10 research outputs found
Transition to Superfluid Turbulence
Turbulence in superfluids depends crucially on the dissipative damping in
vortex motion. This is observed in the B phase of superfluid 3He where the
dynamics of quantized vortices changes radically in character as a function of
temperature. An abrupt transition to turbulence is the most peculiar
consequence. As distinct from viscous hydrodynamics, this transition to
turbulence is not governed by the velocity-dependent Reynolds number, but by a
velocity-independent dimensionless parameter 1/q which depends only on the
temperature-dependent mutual friction -- the dissipation which sets in when
vortices move with respect to the normal excitations of the liquid. At large
friction and small values of 1/q < 1 the dynamics is vortex number conserving,
while at low friction and large 1/q > 1 vortices are easily destabilized and
proliferate in number. A new measuring technique was employed to identify this
hydrodynamic transition: the injection of a tight bundle of many small vortex
loops in applied vortex-free flow at relatively high velocities. These vortices
are ejected from a vortex sheet covering the AB interface when a two-phase
sample of 3He-A and 3He-B is set in rotation and the interface becomes unstable
at a critical rotation velocity, triggered by the superfluid Kelvin-Helmholtz
instability.Comment: Short review; to be published in Journal of Low Temperature Physics
(2006
Hierarchical Equations of Motion Approach to Quantum Thermodynamics
We present a theoretical framework to investigate quantum thermodynamic
processes under non-Markovian system-bath interactions on the basis of the
hierarchical equations of motion (HEOM) approach, which is convenient to carry
out numerically "exact" calculations. This formalism is valuable because it can
be used to treat not only strong system-bath coupling but also system-bath
correlation or entanglement, which will be essential to characterize the heat
transport between the system and quantum heat baths. Using this formalism, we
demonstrated an importance of the thermodynamic effect from the tri-partite
correlations (TPC) for a two-level heat transfer model and a three-level
autonomous heat engine model under the conditions that the conventional quantum
master equation approaches are failed. Our numerical calculations show that TPC
contributions, which distinguish the heat current from the energy current, have
to be take into account to satisfy the thermodynamic laws.Comment: 9 pages, 4 figures. As a chapter of: F. Binder, L. A. Correa, C.
Gogolin, J. Anders, and G. Adesso (eds.), "Thermodynamics in the quantum
regime - Recent Progress and Outlook", (Springer International Publishing
Safety and radiation protection in waste management. Final report of the Nordic Nuclear Safety Research Project SOS-3
Heat current characteristics in nanojunctions with superconducting baths
As a fundamental requisite for thermotronics, controlling heat flow has been a longstanding quest in solid state physics. Recently, there has been a lot of interest in nanoscale hybrid systems as possible candidates for thermal devices. In this context, we study the heat current in the simplest hybrid device of a two level system weakly coupled to two heat baths. We use the reduced density matrix approach together with a simple Born-Markov approximation to calculate the heat current in the steady state. We consider different kinds of reservoirs and show that the nature of the reservoir plays a very important role in determining the thermal characteristics of the device. In particular, we investigate the effectiveness of a conventional superconductor as a reservoir with regard to manipulating the heat current. In the emergent temperature characteristics, we find that superconductivity in the reservoirs leads to enhanced thermal currents and that the superconducting phase transition is clearly visible in the heat current. We observe negative differential thermal conductance and a pronounced rectification of the heat current, making this 9 a good building block for a quantum thermal diode.Departamento Administrativo de Ciencia, Tecnología e Innovación [CO] Colciencias1115-569-34912n