Two-Stage Kondo Effect in a Quantum Dot at a High Magnetic Field

Abstract

We report a strong Kondo effect (Kondo temperature ϳ4 K) at high magnetic field in a selective area growth semiconductor quantum dot. The Kondo effect is ascribed to a singlet-triplet transition in the ground state of the dot. At the transition, the low-temperature conductance approaches the unitary limit. Away from the transition, for low bias voltages and temperatures, the conductance is sharply reduced. The observed behavior is compared to predictions for a two-stage Kondo effect in quantum dots coupled to single-channel leads. DOI: 10.1103/PhysRevLett.88.126803 PACS numbers: 73.23.Hk, 72.15.Qm The observation of the Kondo effect in quantum dots [1 -3] has led to an increased experimental and theoretical interest in this many-body phenomenon. Unlike the conventional case of bulk metals containing magnetic impurities [4], quantum dots [5] offer the possibility to study the Kondo effect at the level of a single artificial magnetic impurity [6], allowing one to tune different parameters. Experiments on quantum dots have also revealed novel Kondo phenomena that have no analog in bulk-metal systems. In particular, multilevel Kondo effects have been studied both theoretically [7][8][9][10][11][12] and experimentally [13][14][15] that differ substantially from the ordinary case of a spin-1͞2 Anderson impurity.In this Letter, we present results on a strong Kondo effect in a lateral quantum dot at high magnetic field. We associate the Kondo effect with a magnetically induced crossing between a spin-singlet and a spin-triplet ground state [9 -14]. In contrast to the results for a vertical semiconductor quantum dot [13] and for a carbon nanotube dot [14], we find a sharp reduction of the conductance at low bias voltage, V SD , and temperature, T. We ascribe the different behavior to the number of channels in the leads which couple to the states in the dot. In lateral dots, tunnel barriers are obtained by successively pinching off the propagating channels. Coulomb blockade develops when the last channel is nearly pinched off. Therefore, only one channel in each lead is coupled to the dot [16]. In vertical dots, however, the tunnel barrier characteristics are determined by the growth parameters, i.e., by the thickness of the different semiconductor materials forming the heterostructure and their relative conduction band offsets. In this case, more than one conducting channel can effectively couple to the dot states. The same is true for carbon nanotubes connected to metal leads. Our results, in combination with previous findings [13,14], show that screening of higher spin states (S $ 1͒ depends strongly on the number of channels coupled to the (artificial) magnetic impurity. Comparison is made to recent theoretical studies on quantum dots in Refs. [16,17], which are partly inspired by the experimental work presented here.Our device (Figs. 1a and 1b) consists of a lateral quantum dot [18] with a nominal diameter ϳ300 nm that is further decreased by application of a negative voltage to the top gate electrode (Fig. 1a)....

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

“…One can then naturally wonder about the fate of a singlet to degenerate triplet crossing in the presence of a single screening channel [26,27,28,29,30,31,32,33,34], which can in principle be expected for lateral quantum dots [35,36] and molecular devices [34].…”

Kondo effect and spin filtering in triangular artificial atoms

“…We have also sug-gested using triangular atoms to study systematically the singlet-triplet transition described in Ref. [4] and apparently observed in Ref. [11].…”

“…The role of orbital degeneracy in the formation of strongly correlated states in artificial atoms has not received the scrutiny it deserves, perhaps because orbital degeneracy is so hard to control experimentally in these systems. Accidental degeneracies may result in spin S > 1/2 states with interesting physical properties [3], and in artificial atoms with almost degenerate states a singlet-triplet transition may also occur, giving rise to interesting non-monotonic behavior in the temperature dependence of the conductance [3][4][5]. However, a typical artificial atom, unlike its namesakes, has no spatial symmetries and hence no orbital degeneracies.…”

“…The Kondo temperatures are in the measurable range. The triangular atom also provides a tool to systematically study the singlet-triplet transitions observed in recent experiments [4,11]. …”

“…Distorting the shape of the TA with an external gate would gradually split the degenerate Γ 3 states. Therefore, in this geometry one could systematically study the singlettriplet transition without applying any external magnetic field [4,11].…”