Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Egyptian journal of solids

The Break Down of Semiclassical Density Matrix of a Particle Moving in Barrier Potential at Temperature T

Published Dec 1, 2004• 0 citations
Publisher

Abstract

The nonlinear barrier potential with bound states is presented. Theequilibrium density matrix of a particle moving at temperature T in thisnonlinear barrier potential field is determined. The exact density matrix iscompared with the result of the path integral approach in the semiclassicalapproximation is found to be sufficient at high temperatures while at lowtemperatures the fluctuation paths may have a caustic depending ontemperature and end points. Near the caustics, the divergence of the simplesemicalssical approximation of the density matrix is removed by a nonlinearfluctuation potential. For opaque barriers, the improved semiclassicalapproximation is compulsory required.

DOI
10.21608/ejs.2004.149311

Related papers

Exact and semiclassical density matrix of a particle moving in a barrier potential with bound states

Franz Josef Weiper, Joachim Ankerhold, Hermann Grabert

We present a barrier potential with bound states that is exactly solvable and determine the eigenfunctions and eigenvalues of the Hamiltonian. The equilibrium density matrix of a particle moving at temperature T in this nonlinear barrier potential field is determined. The exact density matrix is compared with the result of the path integral approach in the semiclassical approximation. For opaque barriers the simple semiclassical approximation is found to be sufficient at high temperatures while at low temperatures the fluctuation paths may have a caustic depending on temperature and endpoints. Near the caustics the divergence of the simple semiclassical approximation of the density matrix is removed by a nonlinear fluctuation potential. For opaque barriers the improved semiclassical approximation is again in agreement with the exact result. In particular, bound states and the form of resonance states are described accurately by the semiclassical approach.

arXiv:chem-ph/9602002•The Journal of Chemical Physics•May 15, 1996•6 citations

Dissipative quantum systems with a potential barrier. II. Dynamics near the barrier top

Joachim Ankerhold, Hermann Grabert

We study the real time dynamics of a quantum system with a potential barrier coupled to a heat-bath environment. The time evolution of the density matrix starting from a general initial state is evaluated explicitly in the semiclassical approximation. As the temperature is decreased below a critical temperature ${\mathit{T}}_{\mathit{c}}$, large quantum fluctuations render the harmonic approximation near the barrier top insufficient and a caustic arises. As a consequence, anharmonicities of the potential become essential, even in the close vicinity of the barrier top, and the semiclassical density matrix has to be evaluated beyond the Gaussian approximation. The range of validity with respect to temperature and time of the semiclassical analytical results is discussed in detail. We illustrate the outcome of the theory with an explicit example.

Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics•Nov 1, 1995•13 citations

Semiclassical time evolution of the density matrix and tunneling

Joachim Ankerhold, Hermann Grabert

The time dependent density matrix of a system with potential barrier is studied using path integrals. The characterization of the initial state, which is assumed to be restricted to one side of the barrier, and the time evolution of the density matrix lead to a threefold path integral which is evaluated in the semiclassical limit. The semiclassical trajectories are found to move in the complex coordinate plane and barrier penetration only arises due to fluctuations. Both the form of the semiclassical paths and the relevant fluctuations change significantly as a function of temperature. The semiclassical analysis leads to a detailed picture of barrier penetration in the real time domain and the changeover from thermal activation to quantum tunneling. Deep tunneling is associated with quasizero modes in the fluctuation spectrum about the semiclassical orbits in the long time limit. The connection between this real time description of tunneling and the standard imaginary time instanton approach is established. Specific results are given for a double well potential and an Eckart barrier.

arXiv:cond-mat/0001380•Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics•Apr 1, 2000•12 citations

Quantum tunneling and the semiclassical real time dynamics of the density matrix

Joachim Ankerhold, Hermann Grabert

The time evolution of the density matrix of a system with potential barrier is determined using path integrals. Describing the quantum dynamics in terms of the semiclassical motion in the complex coordinate plane, a detailed picture of barrier penetration in the time domain and the change-over from thermal activation to quantum tunneling is gained. Deep tunneling is shown to arise from specific fluctuations about the semiclassical real time trajectories.

Europhysics Letters (EPL)•Aug 1, 1999•18 citations

Dissipative quantum systems with a potential barrier. III. Steady state nonequilibrium flux and reaction rate

Joachim Ankerhold, Hermann Grabert

We study the real time dynamics of a dissipative quantum system in a metastable state which may decay by crossing a potential barrier. Starting from an initial state where the system is in thermal equilibrium on one side of the barrier, the time evolution of the density matrix is evaluated analytically in the semiclassical approximation for coordinates near the barrier top. In a region about a critical temperature ${\mathrm{T}}_{\mathrm{c}}$ large quantum fluctuations render the harmonic approximation of the potential insufficient and anharmonicities become essential. Accounting for non-Gaussian fluctuation modes, we show that the density matrix approaches a quasistationary state with a constant flux across the barrier. This extends our earlier results [Phys. Rev. E 51, 4267 (1995)] on the quantum generalization of the Kramers flux state to the region about ${\mathrm{T}}_{\mathrm{c}}$. By matching the flux state onto the equilibrium state on one side of the barrier, we determine the decay rate out of the metastable state. The rate constant shows a changeover from thermally activated decay to quantum tunneling for temperatures below ${\mathrm{T}}_{\mathrm{c}}$.

Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics•Feb 1, 1997•9 citations

Semiclassical tunneling in real time: Wave-packet dynamics in static and driven barrier potentials

Markus Saltzer, Joachim Ankerhold

We study to what extent propagation of wave packets through potential barriers can be described semiclassically in real time. By analyzing the semiclassical approximation to the exact path-integral representation of the quantum-mechanical time evolution operator, we show that in the long-time sector it is dominated by quasiclassical fluctuations associated with caustics. Contrary to the description of tunneling in imaginary times, simple tunneling paths even for complex energies do not exist. Our findings lead to an extended semiclassical propagation scheme for wave-packet dynamics which is numerical efficient and accurately describes deep incoherent tunneling through static and externally driven barrier potentials.

Physical Review A•Oct 13, 2003•20 citations
The nonlinear barrier potential with bound states is presented. Theequilibrium density matrix of a particle moving at temperature T in thisnonlinear barrier potential field is determined. The exact density matrix iscompared with the result of the path integral approach in the semiclassicalapproximation is found to be sufficient at high temperatures while at lowtemperatures the fluctuation paths may have a caustic depending ontemperature and end points. Near the caustics, the divergence of the simplesemicalssical approximation of the density matrix is removed by a nonlinearfluctuation potential. For opaque barriers, the improved semiclassicalapproximation is compulsory required.
We present a barrier potential with bound states that is exactly solvable and determine the eigenfunctions and eigenvalues of the Hamiltonian. The equilibrium density matrix of a particle moving at temperature T in this nonlinear barrier potential field is determined. The exact density matrix is compared with the result of the path integral approach in the semiclassical approximation. For opaque barriers the simple semiclassical approximation is found to be sufficient at high temperatures while at low temperatures the fluctuation paths may have a caustic depending on temperature and endpoints. Near the caustics the divergence of the simple semiclassical approximation of the density matrix is removed by a nonlinear fluctuation potential. For opaque barriers the improved semiclassical approximation is again in agreement with the exact result. In particular, bound states and the form of resonance states are described accurately by the semiclassical approach.
We study the real time dynamics of a quantum system with a potential barrier coupled to a heat-bath environment. The time evolution of the density matrix starting from a general initial state is evaluated explicitly in the semiclassical approximation. As the temperature is decreased below a critical temperature Tc{\mathit{T}}_{\mathit{c}}Tc​, large quantum fluctuations render the harmonic approximation near the barrier top insufficient and a caustic arises. As a consequence, anharmonicities of the potential become essential, even in the close vicinity of the barrier top, and the semiclassical density matrix has to be evaluated beyond the Gaussian approximation. The range of validity with respect to temperature and time of the semiclassical analytical results is discussed in detail. We illustrate the outcome of the theory with an explicit example.
The time dependent density matrix of a system with potential barrier is studied using path integrals. The characterization of the initial state, which is assumed to be restricted to one side of the barrier, and the time evolution of the density matrix lead to a threefold path integral which is evaluated in the semiclassical limit. The semiclassical trajectories are found to move in the complex coordinate plane and barrier penetration only arises due to fluctuations. Both the form of the semiclassical paths and the relevant fluctuations change significantly as a function of temperature. The semiclassical analysis leads to a detailed picture of barrier penetration in the real time domain and the changeover from thermal activation to quantum tunneling. Deep tunneling is associated with quasizero modes in the fluctuation spectrum about the semiclassical orbits in the long time limit. The connection between this real time description of tunneling and the standard imaginary time instanton approach is established. Specific results are given for a double well potential and an Eckart barrier.
We study the real time dynamics of a dissipative quantum system in a metastable state which may decay by crossing a potential barrier. Starting from an initial state where the system is in thermal equilibrium on one side of the barrier, the time evolution of the density matrix is evaluated analytically in the semiclassical approximation for coordinates near the barrier top. In a region about a critical temperature Tc{\mathrm{T}}_{\mathrm{c}}Tc​ large quantum fluctuations render the harmonic approximation of the potential insufficient and anharmonicities become essential. Accounting for non-Gaussian fluctuation modes, we show that the density matrix approaches a quasistationary state with a constant flux across the barrier. This extends our earlier results [Phys. Rev. E 51, 4267 (1995)] on the quantum generalization of the Kramers flux state to the region about Tc{\mathrm{T}}_{\mathrm{c}}Tc​. By matching the flux state onto the equilibrium state on one side of the barrier, we determine the decay rate out of the metastable state. The rate constant shows a changeover from thermally activated decay to quantum tunneling for temperatures below Tc{\mathrm{T}}_{\mathrm{c}}Tc​.
We study to what extent propagation of wave packets through potential barriers can be described semiclassically in real time. By analyzing the semiclassical approximation to the exact path-integral representation of the quantum-mechanical time evolution operator, we show that in the long-time sector it is dominated by quasiclassical fluctuations associated with caustics. Contrary to the description of tunneling in imaginary times, simple tunneling paths even for complex energies do not exist. Our findings lead to an extended semiclassical propagation scheme for wave-packet dynamics which is numerical efficient and accurately describes deep incoherent tunneling through static and externally driven barrier potentials.