Magnetically Tuned Kondo Effect in a Molecular Double Quantum Dot: Role of the Anisotropic Exchange

Abstract

We investigate theoretically and experimentally the singlet-triplet Kondo effect induced by a magnetic field in a molecular junction. Temperature dependent conductance, G(T ), is calculated by the numerical renormalization group, showing a strong imprint of the relevant low energy scales, such as the Kondo temperature, exchange and singlet-triplet splitting. We demonstrate the stability of the singlet-triplet Kondo effect against weak spin anisotropy, modeled by an anisotropic exchange. Moderate spin anisotropy manifests itself by lowering the Kondo plateaus, causing the G(T ) to deviate from a standard temperature dependence, expected for a spin-half Kondo effect. We propose this scenario as an explanation for anomalous G(T ), measured in an organic diradical molecule coupled to gold contacts. We uncover certain new aspects of the singlet-triplet Kondo effect, such as coexistence of spin-polarization on the molecule with Kondo screening and nonperturbative parametric dependence of an effective magnetic field induced by the leads.

Time-dependent spintronic anisotropy in magnetic molecules

“…Additionally, when transverse anisotropy component is considerable, quantum tunneling of magnetization may occur [16][17][18][19]. The transport properties of magnetic molecules have already been extensively studied [20][21][22][23][24][25][26][27][28][29][30][31], including the influence of the Kondo effect [32][33][34][35][36] in the strong coupling regime [37][38][39][40][41][42][43]. However, when the physics of SMM systems incorporates spintronics, new prominent effects are revealed, including switching with spin-polarized currents, Berry-phase blockade or spintronic anisotropy among many others [44][45][46][47][48][49][50][51][52].…”

Poor man’s scaling and Lie algebras

“…One of such approaches was the so called poor man's scaling, pioneered by Anderson [3]. Models, similar to the one mentioned above, describe magnetic ions in a crystalline electric field [4,5], tunnelling centres [6] and system of quantum dots [7][8][9][10][11][12][13]. It is known that the anisotropy can substantially change the physics of the model in comparison with the isotropic case [14][15][16][17][18][19][20][21].…”