Singlet–triplet transition in a single-electron transistor at zero magnetic field

Abstract

We report sharp peaks in the differential conductance of a single-electron transistor (SET) at low temperature, for gate voltages at which charge fluctuations are suppressed. For odd numbers of electrons we observe the expected Kondo peak at zero bias. For even numbers of electrons we generally observe Kondo-like features corresponding to excited states. For the latter, the excitation energy often decreases with gate voltage until a new zero-bias Kondo peak results. We ascribe this behavior to a singlet-triplet transition in zero magnetic field driven by the change of shape of the potential that confines the electrons in the SET.

Universal transport signatures in two-electron molecular quantum dots: gate-tunable Hund’s rule, underscreened Kondo effect and quantum phase transitions

“…One crucial and common ingredient is the presence of a single active screening channel in the accessible temperature range, which is granted by the asymmetric hybridization between orbital and leads. This is expected to be satisfied in most devices, and indeed gatedependence of the magnetic excitations has been reported in all kinds of quantum dot systems: semi-conducting devices [36], carbon nanotubes [63,65,64], several kinds of molecules [34,67]. The singlet-triplet quantum phase transition [34] imposes however a further experimental requirement, namely a small bare singlet-triplet splitting, in order to allow the gating effect to overcome it.…”

Single-molecule transport in three-terminal devices

“…At negative bias, the third derivative ( figure 5.4(c)) indicates the presence of three peaks. This evolution in a magnetic field, B, is consistent with inelastic cotunneling from a singlet ground state to an excited triplet state (see figure 5.4(b)) [73,74]. The splitting in each peak should be gμ B m s B with m s = ±1.…”

Nonequilibrium transport theory of the singlet-triplet transition: Perturbative approach

“…[36][37][38][39][40] In case of two electrons, on the other hand, one finds a transition (cross-over) between a state, where the two electrons are bound into a singlet, and another state, where the two electrons' spin is aligned into a triplet, which is then completely or partially screened by the spin of the conduction electrons in the leads. 16,17,[41][42][43][44] Here we focus our attention to this second, most interesting regime, also nicknamed as the singlet-triplet (ST) transition. We focus on the most realistic case, 43 where two independent conduction electron channels are coupled to the dot levels, and therefore the singlet-triplet transition is just a cross-over.…”

“…[41][42][43][44] This must be contrasted to the case of a single coupled conduction electron channel, when the transition is a true Kosterlitz-Thouless quantum phase transition. 16,17 Non-equilibrium transport in this latter case has been investigated recently using the so-called non-crossing approximation (NCA). 45 Describing the case of two coupled channels under non-equilibrium conditions represents a theoretical challange.…”

“…8 Moreover, the unprecedented control of these minuscule structures opens new and fascinating possibilities to build and study hybrid structures, [9][10][11][12][13] entangle electron spins, 14 and also to realize and study simple quantum systems in the close vicinity of quantum phase transitions under non-equlibrium conditions. [15][16][17][18][19] Understanding the physical properties of these tiny electronic circuits represents a major challenge to theoretical as well as to experimental physicists. Due to their extremely small size, electron-electron interaction is often dominant in these structures, and moreover, as mentioned before, they are operated under non-equilibrium conditions.…”