Mechanical Control of Spin States in Spin-1 Molecules and the Underscreened Kondo Effect

Parks, J. J.; Champagne, A. R.; Costi, T. A.; Shum, William W.; Pasupathy, Abhay; Neuscamman, Eric; Flores-Torres, Samuel; Cornaglia, P. S.; Aligia, A. A.; Balseiro, C. A.; Chan, Garnet; Abruña, Héctor D.; Ralph, Daniel

doi:10.1126/science.1186874

Cited by 460 publications

(545 citation statements)

References 27 publications

(55 reference statements)

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“…Recent theoretical work [19,45] proposed spin-orbit effects (or more generally magnetic anisotropies) as a possible alternative interpretation for some of these other experiments. We will show however that spin-orbit coupling can be neglected in our particular experiment, which is a crucial requirement [19,45] for the robustness of our observation of Kondo underscreening [18]. Finally, recent experimental and theoretical studies [65,66,67,18,68,69,70] have confirmed several microscopic mechanisms at play in the singlet-triplet quantum phase transition, such as the gate tunable Hund's rule, giving extra strength to our initial interpretation.…”

Section: Experiments In Even Charge Quantum Dots: Gate Control Of Spimentioning

confidence: 99%

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

Florens

Freyn

Roch

et al. 2011

J. Phys.: Condens. Matter

110

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Abstract. We review here some universal aspects of the physics of two-electron molecular transistors in the absence of strong spin-orbit effects. Several recent quantum dots experiments have shown that an electrostatic backgate could be used to control the energy dispersion of magnetic levels. We discuss how the generically asymmetric coupling of the metallic contacts to two different molecular orbitals can indeed lead to a gate-tunable Hund's rule in the presence of singlet and triplet states in the quantum dot. For gate voltages such that the singlet constitutes the (non-magnetic) ground state, one generally observes a suppression of low voltage transport, which can yet be restored in the form of enhanced cotunneling features at finite bias. More interestingly, when the gate voltage is controlled to obtain the triplet configuration, spin S = 1 Kondo anomalies appear at zero-bias, with non-Fermi liquid features related to the underscreening of a spin larger than 1/2. Finally, the small bare singlet-triplet splitting in our device allows to fine-tune with the gate between these two magnetic configurations, leading to an unscreening quantum phase transition. This transition occurs between the non-magnetic singlet phase, where a two-stage Kondo effect occurs, and the triplet phase, where the partially compensated (underscreened) moment is akin to a magnetically "ordered" state. These observations are put theoretically into a consistent global picture by using new Numerical Renormalization Group simulations, taylored to capture sharp finie-voltage cotunneling features within the Coulomb diamonds, together with complementary outof-equilibrium diagrammatic calculations on the two-orbital Anderson model. This work should shed further light on the complicated puzzle still raised by multi-orbital extensions of the classic Kondo problem.

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Section: Experiments In Even Charge Quantum Dots: Gate Control Of Spimentioning

confidence: 99%

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

Florens

Freyn

Roch

et al. 2011

J. Phys.: Condens. Matter

110

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show abstract

“…The presence of a zero-bias Kondo peak still implies a spin-degenerate ground state, and removing an electron must therefore introduce an additional spin, S 2 , which couples ferromagnetically with S 1 to form a triplet ground state, resulting in a spin-1 Kondo effect. 28,29 That the spin has not changed by more than ±1/2 is consistent with the absense of spin blockade. 27 Note that instead of the usual enhancement of the Kondo peak upon approaching the charge degeneracy point a slight depression is observed near V g = 2.0 V in Fig.…”

Section: From the Clear Observation Of A Zeeman Effect [See Figs 2(dmentioning

confidence: 59%

Manipulation of organic polyradicals in a single-molecule transistor

et al. 2012

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Inspired by cotunneling spectroscopy of spin-states in a single OPE5-based molecule, we investigate the prospects for electric control of magnetism in purely organic molecules contacted in a three-terminal geometry. Using the gate electrode, the molecule is reversibly switched between three different redox states, with magnetic spectra revealing both ferromagnetic and antiferromagnetic exchange couplings on the molecule. These observations are shown to be captured by an effective low-energy Heisenberg model, which we substantiate microscopically by a simple valence bond description of the molecule. These preliminary findings suggest an interesting route towards functionalized all-organic molecular magnetism.

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“…Transition-metal magnetic atoms on insulating surfaces, in particular, Mn, 3,4 Co, 5,6 and Fe, [7][8][9] have been the focus of intensive research in the last few years. These have all partially filled d shells, which are highly localized and responsible for the magnetic moment, and extended s-like electrons, which are responsible for the electron conduction.…”

Section: Introductionmentioning

confidence: 99%

Bias asymmetry in the conductance profile of magnetic ions on surfaces probed by scanning tunneling microscopy

2012

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The conductance profiles of magnetic transition-metal atoms, such as Fe, Co, and Mn, deposited on surfaces and probed by a scanning tunneling microscope (STM), provide detailed information on the magnetic excitations of such nanomagnets. In general, the profiles are symmetric with respect to the applied bias. However, a set of recent experiments has shown evidence for inherent asymmetries when either a normal or a spin-polarized STM tip is used. In order to explain such asymmetries, here we expand our previously developed perturbative approach to electron-spin scattering to the spin-polarized case and to the inclusion of out of equilibrium spin populations. In the case of a magnetic STM tip, we demonstrate that the asymmetries are driven by the nonequilibrium occupation of the various atomic spin levels, an effect that is reminiscent of electron spin transfer. In contrast, when the tip is not spin polarized, such a nonequilibrium population cannot be built up. In this circumstance, we propose that the asymmetry simply originates from the transition metal ion density of states and from the uneven electronic coupling between the scattering magnetic atoms and both the tip and the substrate. These effects are included in the formalism as a nonvanishing real component to the spin-scattering self-energy.

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Mechanical Control of Spin States in Spin-1 Molecules and the Underscreened Kondo Effect

Cited by 460 publications

References 27 publications

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

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

Manipulation of organic polyradicals in a single-molecule transistor

Bias asymmetry in the conductance profile of magnetic ions on surfaces probed by scanning tunneling microscopy

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