51 research outputs found
Parameter symmetries of quantum many-body systems
We analyze the occurrence of dynamically equivalent Hamiltonians in the
parameter space of general many-body interactions for quantum systems,
particularly those that conserve the total number of particles. As an
illustration of the general framework, the appearance of parameter symmetries
in the interacting boson model-1 and their absence in the Ginocchio SO(8)
fermionic model are discussed.Comment: 8 pages, REVTeX, no figur
Collective performance of a finite-time quantum Otto cycle
We study the finite-time effects in a quantum Otto cycle where a collective
spin system is used as the working fluid. Starting from a simple one-qubit
system we analyze the transition to the limit cycle in the case of a
finite-time thermalization. If the system consists of a large sample of
independent qubits interacting coherently with the heat bath, the superradiant
equilibration is observed. We show that this phenomenon can boost the power of
the engine. Mutual interaction of qubits in the working fluid is modeled by the
Lipkin-Meshkov-Glick Hamiltonian. We demonstrate that in this case the quantum
phase transitions for the ground and excited states may have a strong negative
effect on the performance of the machine. Reversely, by analyzing the work
output we can distinguish between the operational regimes with and without a
phase transition.Comment: 13 pages, 11 figure
Thermodynamic Analogy for Structural Phase Transitions
We investigate the relationship between ground-state (zero-temperature)
quantum phase transitions in systems with variable Hamiltonian parameters and
classical (temperature-driven) phase transitions in standard thermodynamics. An
analogy is found between (i) phase-transitional distributions of the
ground-state related branch points of quantum Hamiltonians in the complex
parameter plane and (ii) distributions of zeros of classical partition
functions in complex temperatures. Our approach properly describes the first-
and second-order quantum phase transitions in the interacting boson model and
can be generalized to finite temperatures.Comment: to be published by AIP in Proc. of the Workshop "Nuclei and
Mesoscopic Physics" (Michigan State Univ., Oct 2004); 10 pages, 3 figure
Excited-state quantum phase transitions in systems with two degrees of freedom: III. Interacting boson systems
The series of articles [Ann. Phys. 345, 73 (2014) and 356, 57 (2015)] devoted
to excited-state quantum phase transitions (ESQPTs) in systems with
degrees of freedom is continued by studying the interacting boson model of
nuclear collective dynamics as an example of a truly many-body system. The
intrinsic Hamiltonian formalism with angular momentum fixed to is used to
produce a generic first-order ground-state quantum phase transition with an
adjustable energy barrier between the competing equilibrium configurations. The
associated ESQPTs are shown to result from various classical stationary points
of the model Hamiltonian, whose analysis is more complex than in previous cases
because of (i) a non-trivial decomposition to kinetic and potential energy
terms and (ii) the boundedness of the associated classical phase space.
Finite-size effects resulting from a partial separability of both degrees of
freedom are analyzed. The features studied here are inherent in a great
majority of interacting boson systems.Comment: 14 pages, 6 figure
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