Official Resources
- Repository: https://github.com/TwenteQT/TwenteQuantumTransport
- License: GNU General Public License v3.0
Overview
TQT ( Twente Quantum Transport) is a versatile, high-performance Fortran code for simulating spin-dependent electron transport in nanoelectronics and spintronic devices. It specifically targets realistic material systems with disorder (chemical, thermal, magnetic) by employing a Scattering Matrix approach combined with Green's functions. TQT supports various Hamiltonian types, including tight-binding (Slater-Koster) and Muffin-Tin Orbitals (LMTO/EMTO), making it suitable for both model studies and first-principles-based transport calculations in large supercells.
Scientific domain: Spintronics, Quantum Transport, Disordered Systems
Target user community: Researchers in spintronics, nanomagnetism, and device physics
Theoretical Methods
- Landauer-Büttiker Formalism: Calculates conductance and transmission probabilities from the scattering matrix $S$.
- Scattering Matrix (S-matrix): Uses a wavefunction matching technique (wavefront propagation) which is numerically stable for large systems.
- Recursive Green's Functions (RGF): For computing local quantities like charge and spin densities.
- Disorder Averaging: Efficiently handles random disorder via supercell averaging or configuration sampling.
Capabilities
- Transport Properties:
- Spin-dependent conductance ($G_{\uparrow}, G_{\downarrow}$).
- Transmission coeffecients $T(E, \mathbf{k}_{||})$.
- Shot noise and Fano factor.
- Spintronics:
- Spin-Transfer Torque (STT).
- Spin-Orbit Torque (SOT).
- Spin diffusion capability.
- Magnetocrystalline anisotropy (if SOC included).
- System Types:
- Magnetic Tunnel Junctions (MTJs).
- Spin Valves (GMR/TMR).
- Domain Walls and Skrymions.
- Point Contacts.
Key Strengths
- Supercell Scalability: Unlike CPA codes, TQT explicitly treats disorder in large lateral supercells, capturing effects like Anderson localization and diffusive transport.
- Versatility: Unified treatment of TB and MTO Hamiltonians.
- Stability: The S-matrix implementation is extremely stable for long systems where standard transfer matrix methods fail.
- Non-Collinear Magnetism: Full support for non-collinear spin textures (domain walls, spin spirals).
Inputs & Outputs
- Inputs:
- Hamiltonian files (TB or LMTO/EMTO format).
input config file: Geometry, energy range, k-points, disorder settings.
- Outputs:
conductance.dat: Energy/k-resolved transmission.density.dat: Local density of states/charge.currents.dat: Spin/Charge current distributions.
Interfaces & Ecosystem
- Upstream:
- Crary (LMTO code often used at Twente).
- EMTO: Can map EMTO parameters to tight-binding forms.
- Analysis: Python processing tools provided in the repository.
Performance Characteristics
- Computational Cost: $O(N)$ scaling with system length (thanks to RGF/S-matrix).
- Parallelism: MPI parallelization over energy points and transverse k-points ($k_x, k_y$). Efficient for high-throughput screening.
- Memory: Moderate; stores slice-by-slice matrices, avoiding full system Hamiltonian storage.
Limitations & Known Constraints
- Electrostatics: Typically uses a "frozen potential" or simple self-consistency; possibly less rigorous Poisson solving than NEGF-DFT codes like TranSIESTA.
- Basis: Relies on localized orbital descriptions; no plane-wave support.
Comparison with Other Codes
- vs. Kwant: Kwant is Python-based and very flexible for models; TQT is optimized Fortran 95/2003 with deeper support for specific ab initio MTO bases and spintronic observables.
- vs. Smeagol: Smeagol is DFT-NEGF; TQT is often used as a "post-DFT" transport solver on fitted or MTO Hamiltonians, allowing larger system sizes.
Application Areas
- MRAM: Modeling tunnel magnetoresistance in Fe/MgO/Fe junctions.
- Spin Logic: Spin-orbit torque switching simulations.
- Material Science: Scattering by grain boundaries and interface roughness in metals (Cu interconnects).
Community and Support
- Development: Developed at the University of Twente (Kelly, Starikov groups).
- Source: GitHub.
Verification & Sources