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qcl:software_documentation [2017/03/16 09:51] thomas.grange [Working principle] |
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| - | ===== nextnano.QCL - Software documentation ===== | + | ===== nextnano.QCL - General remarks ===== |
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| - | ===== General remarks ===== | + | |
| nextnano.QCL is a console application that is run from within the [[http://www.nextnano.com/products/nextnanomat.php|nextnanomat]] software (GUI). Alternatively, it can be executed from the command line. The input file specifies the device that shall be simulated. | nextnano.QCL is a console application that is run from within the [[http://www.nextnano.com/products/nextnanomat.php|nextnanomat]] software (GUI). Alternatively, it can be executed from the command line. The input file specifies the device that shall be simulated. | ||
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| If you have further questions, see the [[qcl:faq|FAQ]] or contact <support@nextnano.com>. | If you have further questions, see the [[qcl:faq|FAQ]] or contact <support@nextnano.com>. | ||
| - | ===== Working principle ===== | ||
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| - | The code is based on the non-equilibrium Green's functions (NEGF) formalism (also known as the Keldysh, or Kadanoff-Baym formalism). This formalism allows to account for both quantum transport effects (i.e. coherent transport effects, such as resonant tunneling), as well as scattering mechanisms. | ||
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| - | In the NEGF formalism, scattering processes are described in terms of self-energies. Self-energies and Green's functions are calculated in a self-consistent way, as both elastic and inelastic scattering processes are accounted within the the self-consistent Born approximation. | ||
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| - | The code uses **field-periodic boundary condition**. | ||
| - | In this way the simulation accounts for an infinite periodic structure, with a periodic electric field. | ||
| - | Coherent transport between periods is accounted on a length set by ''<Coherence_length_in_Periods>''. | ||
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| - | === Program flow === | ||
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| - | {{ :qcl:flowchart.jpg?400 |}} | ||
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| - | In the beginning of the calculation, the single-band effective mass Schrödinger equation is solved in real space. | ||
| - | The calculated energy levels and wave functions are then used as input to the NEGF algorithm. | ||
| - | The wave functions are termed **modes** and the NEGF algorithm is written in terms of **mode space** and not **real space** to make it computationally more efficient. | ||
| - | The number of QCL periods that are input to this Schrödinger equation are specified in ''<Number_of_lateral_periods_for_band_structure>''. | ||
| - | In general, the core of the NEGF algorithm should be rather independent of this number, e.g. | ||
| - | <code> | ||
| - | <Number_of_lateral_periods_for_band_structure> 4 | ||
| - | </Number_of_lateral_periods_for_band_structure> | ||
| - | </code> | ||
| - | should lead to very similar results compared to a value of ''5'' but the numerical values might differ slightly. | ||
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| - | {{ :qcl:ldos_emin_emax_okay.jpg?200 |}} | ||
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| - | As a second step, the scattering coupling terms are calculated for each of the accounted mechanism (optical and acoustic phonons, charged impurities, interface roughness, alloy disorder). | ||
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| - | Then, the main part of the calculation consists in the self-consistent NEGF solver. Starting from an initial guess of the Green's functions, the self-energies are calculated. The Green's functions are then calculated iteratively. | ||
| - | Simultaneously, the mean-field electrostatic potential is calculted self-consistenly (Poisson's equation). Such iterations are made until convergence is reached for the Green's functions as well as for the calculated current. | ||
| - | From the Green's functions solution, the gain is then calculated (if requested in the input file). | ||
qcl/software_documentation.txt · Last modified: 2017/07/24 12:51 by thomas.grange
