arXiv subjects

S. P. Kou

Publications and source records attributed to S. P. Kou.

3 recordsLinked to original sources

arXiv2022

General Exceptional Points

Exceptional points are interesting physical phenomena in non-Hermitian physics at which the eigenvalues are degenerate and the eigenvectors coalesce. In this paper, we find that the universal feature of arbitrary non-Hermitian two level systems with singularities is basis defectiveness rather than energy degeneracy or state coalescence. This leads to the discovery of general exceptional points (GEPs). For GEPs, more subtle structures (e.g., Bloch peach), additional classification, and' 'hidden" quantum phase transitions are explored. By using the topologically protected subspace from two edge states in the non-Hermitian SSH model as an example, we illustrate the physical properties of different types of GEPs.

quant-ph↗

arXiv1999

Macroscopic quantum coherence in mesoscopic ferromagnetic systems

In this paper we study the Macroscopic Quantum Oscillation (MQO) effect in ferromagnetic single domain magnets with a magnetic field applied along the hard anistropy axis. The level splitting for the ground state, derived with the conventional instanton method, oscillates with the external field and is quenched at some field values. A formula for quantum tunneling at excited levels is also obtained. The existence of topological phase accounts for this kind of oscillation and the corresponding thermodynamical quantities exhibit similar interference effects which resembles to some extent the electron quantum phase interference induced by gauge potential in the Aharonov-Bohm effect and the $Θ$-vacuum in Yang-Mills field theory..

cond-mat.mes-hall↗

arXiv1999

Crossover from thermal hopping to quantum tunneling in Mn_{12}Ac

The crossover from thermal hopping to quantum tunneling is studied. We show that the decay rate $Γ$ with dissipation can accurately be determined near the crossover temperature. Besides considering the Wentzel-Kramers-Brillouin (WKB) exponent, we also calculate contribution of the fluctuation modes around the saddle point and give an extended account of a previous study of crossover region. We deal with two dangerous fluctuation modes whose contribution can't be calculated by the steepest descent method and show that higher order couplings between the two dangerous modes need to be taken into considerations. At last the crossover from thermal hopping to quantum tunneling in the molecular magnet Mn_{12}Ac is studied.

cond-mat.mes-hall↗