16 research outputs found
Thermal Uhlmann phase in a locally driven two-spin system
We study the geometric Uhlmann phase of mixed states at finite temperature in
a system of two coupled spin- particles driven by a magnetic field
applied to one of the spins. In the parameter space of temperature and
coupling, we show the emergence of two topological Uhlmann phase transitions
when the magnetic field evolves around the equator, where a winding number can
characterize each temperature range. For small couplings, the width of the
temperature gap of the non-trivial phase is roughly the critical temperature
of one-dimensional fermion systems with two-band Hamiltonians. The first
phase transition in the low-temperature regime and small values of the coupling
corresponds to the peak of the \textit{Schottky anomaly} of the heat capacity,
typical of a two-level system in solid-state physics involving the ground and
first excited states. The second phase transition occurs at temperatures very
close to the second maximum of the heat capacity associated with a multilevel
system. We also derive analytical expressions for the thermal Uhlmann phase for
both subsystems, showing that they exhibit phase transitions. In the driven
subsystem, for minimal , a topological phase transition phase appears at
again. However, for larger values of , the transitions occur at lower
temperature values, and they disappear when the coupling reaches a critical
value . The latter is not the case for the undriven subsystem, where at
low temperatures, a single phase transition occurs at . Nevertheless, as
the temperature rises, we demonstrate the emergence of two phase transitions
defining a coupling gap, where the phase is non-trivial and vanishes as the
temperature reaches a critical value.Comment: 11 pages, 13 figure
A pair of particles in inertial microfluidics: effect of shape, softness, and position
Lab-on-a-chip devices based on inertial microfluidics have emerged as a promising technique to manipulate particles in a precise way. Inertial microfluidics exploits internal hydrodynamic forces and the mechanical structure of particles to achieve separation and focusing. The article focuses on the hydrodynamic interaction of two particles. This will help to develop an understanding of the dynamics of particle trains in inertial microfluidics, which are typical structures in multi-particle systems. We perform three-dimensional lattice Boltzmann simulations combined with the immersed boundary method to unravel the dynamics of various mono- and bi-dispersed pairs in inertial microfluidics. We study the influence of different starting positions for mono- and bi-dispersed pairs. We also change their deformability from relatively soft to rigid and choose spherical and biconcave particle shapes. The observed two-particle motions in the present work can be categorized into four types: stable pair, stable pair with damped oscillations, stable pair with bounded oscillations, and unstable pair. We show that stable pairs become unstable when increasing the particle stiffness. Furthermore, a pair with both capsules in the same channel half is more prone to become unstable than a pair with capsules in opposite channel halves.DFG, 163436311, SFB 910: Kontrolle selbstorganisierender nichtlinearer Systeme: Theoretische Methoden und AnwendungskonzepteTU Berlin, Open-Access-Mittel – 202